I. It is now universally admitted that the cases are extremely numerous, in which diver sities of age have led to the establishment of species which have no existence in nature ; the forms thus distinguished being those of the same species in different grades of de velopment. The more our knowledge of the history of the lower tribes of animals in creases, the more is it found that metamor phosis is with them the rule, and not the ex ception ; so that the cases seem comparatively rare, in which an invertebrated animal at its emersion from the egg possesses the cha racters that serve to distinguish it in its adult condition. And just as the larva, pupa, and imago states of any are all compre hended in a complete account of the species, so must we rank the extraordinary diversities of form presented by the Medusa' or the Balani, in the early period of their lives, as coming within the limits of their specific definitions. It is obvious that this source of fallacy can only be completely avoided, when we have obtained an acquaintance with the whole his tory of the life of any individual, from its com mencement to its dissolution, and are thus enabled to say positively what are, and what are not, alterations producible by age. Where this knowledge cannot be acquired, the only safe basis on which the naturalist can pro ceed, is that which is derived from a know ledge of these phenomena as presented in the most nearly allied forms; and yet this often fails, as in the case of the Astacus fiuvia tills (river cray-fish), and Gecarcinus (land crab), which undergo no change that can be called a metamorphosis, notwithstanding that in all other Macrourous and Brachyourous Decapods yet observed, a real metamorphosis takes place. Even in the case of extinct species, the history of whose life can never become known to us by any other means than by the preservation of their remains in different stages of growth, the careful com parison of a sufficient series of these remains will sometimes establish a strong probability, if not a positive certainty, as to their mutual relationship : thus, M. Barrande, of Prague, has succ.s:iect in showing that it is next to certain that no fewer than eighteen forms of Trilobites, which have been described as dis tinct species and ranked under ten different genera, are really the successive forms of one and the same species ; the differences which they present both in size and conformation being analogous to those that we see in the existing tribes most nearly allied to them, and the whole series constituting one con tinuous succession. Instances in which new species have been erected among the higher classes of animals, especially among Birds, for the reception of individuals whose differences were only seasonal, have been so frequently recorded, that it is sufficient here to mention them. It is obvious that such errors can only be corrected by a knowledge of the seasonal changes which the species is liable to undergo. Of this source of difficulty in the discrimination of species, we need take no account in our future inquiries ; for although, in the Orang and Chimpanzee, the alteration in the conformation of the cranium which takes place at the period of second dentition, is so very decided, that it formerly gave rise to much confusion, which has only disap peared before a fuller knowledge of the history of these animals, yet no change of such magnitude occurs in Man ; and of the of change which does take place in the several races of mankind, between infancy and old age, there is seldom much difficulty in collecting information.
2. The influence of external conditions in modifying the conformation both of Plants and Animals, is a question of fundamental importance in the determination of the value of specific characters. In this respect there is a very extraordinary diversity among the several races of living beings, even among those which are most nearly allied to each other ; for whilst some possess such a capacity for variation, that they are easily influenced by changes in external conditions, and can, in consequence, readily adapt themselves to these, to others this capacity seems altogether denied. It is from this circumstance that we find particular species, of plants, as well as of animals, restricted to particular conditions in regard to climate, food, &c., their constitu tions not being able to adapt themselves to any considerable change ; whilst others are more widely dispersed, simply because their constitutions can accommodate themselves to alterations in these conditions. Hence a change of food or climate, to which the latter soon become habituated, is fatal to the former. We see this difference well marked in the Feline tribe ; for whilst the greater part of the larger species, such as the lion and tiger, are inhabitants of tropical regions, and cannot endure the winter's cold, even in the temperate zone, the domestic cat follows man in almost all his wanderings, and can sustain extremes of heat and cold as well as he can himself. This accommodation is effected by a change in the organism itself, of which evident indications are frequently presented, even within the course of a short time thus, sheep transported from this country to the West Indies soon loose their covering of thick wool, and acquire in the place of it a short, fine hair, shining and smooth, like that of the goat in his best state, so that after a few years the sheep can scarcely be distinguished from the goats, save by their general con formation ; and in this, too, from the usual absence of any considerable accumulation of fat in the bodies of the sheep, there is not nearly so much difference between the two races, as there is in temperate climates.
The continued action of the same cir cumstances for a few generations, gives in creased permanence to the new characters of the breed, so that acquired peculiarities of conformation become congenital. Thus, Sir C. Lyell mentions* that some Englishmen engaged in condUcting the mining operations of the Real del Monte Company in Mexico, carried out with them some greyhounds of the best breed, to hunt the hares which abound in that country. The great platform, which is the scene of sport, is at an elevation of about 9000 feet above the level of the sea, and the mercury in the barometer stands habitually at the height of about 19 inches.
It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere, and before they could come up with their prey, they lay down gasping for breath ; but these same animals have produced whelps, which have grown up, and are not in the least degree incommoded by the want of density in the air, but run down the hares with as much ease as the fleetest of their race in this country. In fact, it can
not be reasonably questioned, if the history of the domesticated races of animals be fairly considered, that changes in external condi tions are capable of exerting a very decided influence upon the physical form, the habits and instincts, and the various functions of life, in species possessing this adaptiveness. The variations thus induced extend to con siderable modifications in the external aspect, such as the colour, the texture, and the thickness of the external covering; to the structure of limbs and proportional size of parts ; to the relative development of the organs of the senses and of the psychical powers, involving changes in the form of the cranium, and to acquired propensities, which, within certain limits (depending, it would appear, on their connexion with the natural habits of the species) may become hereditary. Of the changes in psychical characters thus induced by external circumstances, the fol lowing are trustworthy examples. We are informed by M. Roulin (to whose researches on the changes which the domesticated races, introduced by the Spaniards into South Ame rica, have subsequently undergone, we shall frequently have occasion to refer), that a race of dogs employed for hunting deer in the platform of Santa Fe, in Mexico, is distin guished by the peculiar mode in which they attack their game. This consists in seizing the animal by the belly, and overturning it by a sudden effort, taking advantage of the moment when the body of the deer rests only upon the fore-legs ; the weight of the animal thus thrown over, being often six times that of its antagonist. Now, the dog of pure breed inherits a disposition to this kind of chase, and never attacks a deer from before while running; and even should the deer, not perceiving him, come directly upon him, the dog steps aside, and makes his assault upon the flank. On the other hand, European dogs, though of superior strength and general sagacity, are destitute of this instinct ; and for want of similar precautions, they are often killed by the deer on the spot, the cervical vertebrm being dislocated by the violence of the shock.* A new instinct has also become hereditary in a mongrel race of dogs, employed by the inhabitants of the banks of the Magda lena almost exclusively in hunting the white lipped peccari. The address of these dogs consists in restraining their ardour and attach ing themselves to no individual in particular, but keeping the whole in check. Now, among these dogs, some are found, which, the very first time they are taken to the woods, are acquainted with this mode of attack ; whereas a dog of another breed starts forward at once, is surrounded by the peccari, and, whatever may be his strength, is destroyed in a moment. The fixed and deliberate stand of the pointer, again, whether taught by the agency of man, or a habit engendered, like the preceding, by the force of circumstances, is so intimately con nected with the constitution of the race, that occasionally it becomes hereditary ; a young pointer, taken into the field for the first time, being often observed to perform its duty as well as its long-trained seniors. A still more remarkable example of the transmission of acquired psychical peculiarities, is afforded by the case of the retriever, a breed of dogs which has been trained to keep close to the sportsman until he has fired, and then to go in search of the game which he has wounded or brought down. It is obvious that this habit could only have been taught by the agency of man ; and yet it has been frequently observed that a young retriever, on the very first occasion of being taken into the field, has conducted itself as well, and brought back game with as much steadiness, as older dogs which had been schooled into the same ma nceuvre by means of the whip and collar.* No really philosophical botanist or zoolo gist, then, should venture to establish specific distinctions between two races, otherwise than provisionally, until he has been able to assure himself that the one may not be con verted into the other by a change in external conditions. Characters which are of the most trivial kind in themselves, may be valid grounds of specific distinction, if they are not liable to be thus affected ; on the other hand, characters which would be accepted in one group as sufficient for the separation of ge nera, may be found totally inadequate in an other for the discrimination of species, being liable to modification under a very slight change of external conditions. Every one is familiar with the changes which have been in duced in plants by cultivation, —how a " sin gle " flower is converted into a " double " one ; how the spines, prickles, and thorns covering the surface may be obliterated (a change which was fancifully, but not impro perly, termed by Linnmus the taming of wild fruits "); and how the wavy leaves may be come thick and fleshy (as in the change of the Brassica oleracea into the cabbage), or the slender flower-stalks may be converted into a substantial mass (as in the conversion of the same plant into the cauliflower). These changes do not, as some have alleged, throw the least doubt on the " permanence of spe cies," or favour the doctrine of " transmuta tion " in the slightest degree ; for however wide may be limits of variation, each species has its limits; and so far from having per manently advanced, under the influence of cultivation, to a superior type of structure, the plants thus modified will all return to their original form, when subjected to their original conditions. Numerous instances might be cited from the British Flora alone, in which the most experienced botanists are in dis agreement upon the question of specific unity or diversity, simply because they have not yet ascertained the limits to the variations which the same plant may present, when growing under a variety of external conditions ; and the difficulty is yet greater when British and Continental species are compared, the variety of external conditions being greater, and the amount of allowance which should be made for their influence being known in but a few cases. The same may be said of Animals ; particularly of those on which the influence of domestication has been brought to bear. Upon this point, however, we shall defer en larging, until the next head of the inquiry has been considered.