That the species of the mineralogist and the botanist should be owing to influences so different as is implied by the opera tion of a second cause, and the direct interference of a first cause, is not probable. The nature of the forces operating in the production of a lichen may not be so clearly understood as those which arranged the atoms of the crystal on which the lichen spreads. Pouchet has contributed the most valuable evidence as to the fact and mode of the production by external influences of species of With regard to the species of higher organisms, distinguishable as plants and animals, their origin is as yet only matter of speculation.
Buffont regarded varieties as particular alterations of species, which illustrated the mutability of species themselves. The so-called varieties of a species, species of a genus, genera of an order, etc., were with him but so many evidences of the progressive degrees of change, which had been superinduced by time and successive generations, and chiefly by degrada tion from a primordial type. Applying this principle to the species of which he had given the history in his great work, he believed himself able to reduce them to a very small number of primitive stocks, of which he enumerates "fifteen? Lamarck,t adverting to observed ranges of variation in certain species, affirmed that such variations would proceed and keep pace with the continued operation of the causes pro ducing them ; that such changes of form and structure would induce corresponding changes in actions, and that a change of actions, when habitual, became another cause of altered struc ture ; that the more frequent employment of certain parts or organs leads to a proportional increase of development of such parts ; and that as the increased exercise of one part is usually accompanied by a corresponding disuse of another part, this very disuse, by inducing a proportional degree of atrophy, becomes another element in the progressive mutation of organic forms.
A third theorist§ calls to mind the instances of sudden departure from the specific type, manifested by a malformed or monstrous offspring, and quotes the instances in which such malformations have lived and propagated the deviating structure. He notes also the extreme degrees of change and of complexity of structure undergone by the germ and embryo of a highly organized animal in its progress to maturity. He speculates on the influence of premature birth, or on a some what prolonged fcetation, in establishing the beginning of a specific form different from that of the parent.
Mr. assuming that varieties may arise in a wild species, shows how such deviations from type may tend to adapt a variety to some changes in surrounding conditions, under which it is better calculated to exist, than the type form from which it deviated.
No doubt the type-form of any species is that which is best adapted to the conditions under which such species at the time exists ; and as long as those conditions remain unchanged, so long will the type remain ; all varieties departing therefrom being in the same ratio less adapted to the environing conditions of existence. But, if those con ditions change, then the variety of the species at an antece dent date and state of things will become the type-form of the species at a later date, and in an altered state of things.
Mr. Charles Darwin had, previously to Mr. Wallace, pondered over and worked at this principle, which he illustrates by ingenious suppositions, of which I select the following :—" To give an imaginary example from changes in progress on an island :—let the organization of a canine animal which preyed chiefly on rabbits, but sometimes on bares, become slightly plastic ; let these same changes cause the number of rabbits very slowly to decrease, and the number of hares to increase ; the effect of this would be that the fox or dog would be driven to try to catch more hares ; his organization, however, being slightly plastic, those indi viduals with the lightest forms, longest limbs, and best eye sight, let the difference be ever so small, would be slightly favoured, and would tend to live longer, and to survive during that time of the year when food was scarcest ; they would also rear more young, which would tend to inherit these slight peculiarities. The less fleet ones would be rigidly destroyed. I can see no more reason to doubt that these causes in a thousand generations would produce a marked effect, and adapt the form of the fox or dog to the catching of hares instead of rabbits, than that greyhounds can be improved by selection and careful breeding.* Observation of animals in a state of nature, however, is still required to show their degree of plasticity, or the extent to which varieties do arise ; whereby grounds may be had for judging of the probability of the elastic ligaments and joint structures of a feline foot, for example, being superinduced upon the more simple structure of the toe with the non retractile claw, according to the principle of a succession of varieties in time.