SPECIES, in natural history, a term employed to designate groups inferior to genera (see GEN us), but superior to varieties (see VAIIIETY). In mineralogy, the term is of very arbitrary application, serving only, like class, order, genus, etc., the purpose of classifica tion, although it thus indicates common characters or points of real agreement among minerals. In organic nature, it has usually been regarded as possessing a higher and more definite signification. But no term is more difficult to define. Many definitions have been proposed, but none wholly satisfactory; every attempt at definition involving more or less the adoption of sonic disputed theory. If, for example, a species is regarded as including all the beings which have descended from parents created with the essential characters now belonging to the species, not only is the original creation in that partien lar,form taken for granted, but likewise the impossibility of changes in nature, which some of the most eminent naturalists regard as actually taking place, and the belief of which implies no doubt of the act of creation itself, but only a certain opinion as to some of the laws by which organic nature is governed. To regard species as mere indeter minate and fluctuating groups, capable of indefinite modification in the lapse of ages, is equally to adopt a theory. If a species is defined as containing all the individuals which are capable of intermingling without consequent sterility of progeny, other difficult questions must be decided before the definition can lie adopted as to any classes of erea_ tures,while to many kinds it seems incapable of application, and much that is merely the oretical is involved in it.
Naturalists have very generally regarded species as unchanging throughout the succession of generations, except within narrow and marked limits, and have substan tially adopted the definition of Bunn: " A ttnecics is a constant succession of individuals similar to and capable of reproducing each other." Thus De Candolle, the eminent botanist, says: " We unite under the designation of a species all those individuals that mutually hear to each other so dose a resemblance as to allow of our supposing that they may have proceeded originally from a single being or a single pair." And Cuvier, the great zoologist, describes a species as "a succession of individuals which reproduces and perpetuates itself." Here it may be remarked, that even if the permanence of species implied in these definitions were fully ascertained, and their original creation in their present form admitted as unquestionable, it would by no means follow that we must sup pose every species to have proceeded from a single individual or a single pair. Nor, accepting the Biblical statement concerning the human race, that all mankind are the offspring of a single pair, are we entitled to infer that such has been the case also as to all'animals and plants callable of freely intermingling, and which, therefore, are com monly regarded as forming one species.
But the separate creation and immutability of species are disputed, some naturalists maintaining that species undergo modification, and that existing forms of life have descended by true generation from pre-existing forms. Lamarck was the first to pro claim this doctrine, at least so as to attract much attention, about the beginning of the 19th century. lie held that all species, even including man, are descended from species of inferior organization; while to account for the existence of very simple forms at the present day, be had recourse to the supposition of their spontaneous generation. lie was followed, with greater caution, by Gcoffroy Saint-Hilaire, who regarded what we call species as various degenerations of the same type, but did not believe that the existing species are now undergoing modification. Similar views have since been stated by many authors. But the works which have most strongly direct-el attention to them, and in which they have been most fully advocated, are the the Natural His tory of Creation, by an anonymous author, originally published in 1.'44, and which has since passed through many editions; and Darwin s work On the Or gin of Species by means of Natural .;'eleotion, originally published in 1859. Of the other supporters of these views, the most eminent is prof. Huxley, who, without fully adopting the views either of the author of the Vestiges or of Darwin, advocates " the hypothesis which sup poses the species living at any time to he the result of the gradual modification of pre existing species," and maintains that to suppose each species of plant and animal to have been formed and placed on the surface of the globe at long intervals by a distinct act of creative power, is an assumption "as unsupported by tradition or revelation as it is opposed to the general analogy of nature."
It is impossible for us to do more than very briefly exhibit the principal arguments which have been urged on thisquestion. Lamarck rested much on the well-known effect of use or exercise in strengthening and enlarging an organ, and of disuse in atrophying it. "He conceived that, an animal being brought into new circumstances, and called upon to accommodate itself to these, the exertions which it consequently made to that effect caused the rise of new parts: on the contrary, when new circumstances left cer tain existing parts unused, these parts gradually ceased to exist. Something analogous was, he thought, produced in vegetables, by changes in their nutrition, in their absorp tion and transpiration, and in the quantity of caloric, Pat, air, and moisture which they received. This principle, with time, he deemed sufficient for the advance from the monad to the mammal." ,The author of the Vestiges from whom this account of Lamarck's views is taken, regards him as in error " in giving this adaptive principle too much to do;" and says: "In the present day, we have superior light from geology and physi ology, and hence comes the suggestion of a process analogous to ordinary gestation for advancing organic life through its grades, in the course long but definite space of time, with only a recourse to external conditions'as a means of producing the exterior characters." The author of the Vestiges designates the principle for which be conten,ls that of Progressisce Development, and states it as follows: "The proposition determined on, after much consideration, is, that the several series of animated beings, from the simplest and oldest up to the highest and most recent, are, under the providence of God, the results, first, of an impulse which has been imparted to the fonds of life, advancing them in definite times, by generation, through mrades of organization terminating in the highest dicotyledons and vertebrata, these grades being few in number, and generally marked by intervals of organic character which we find to be a praci ical difficulty in ascer taining affinities; second, of another impulse connected with the vital forces. ten,ling, u. the course of generations, to modify organic structures in accordance with external cir cumstances, as food, the nature of the habitat, and the meteoric agencies, these being the ' adaptations' of the natural theologian." He further regards the nucleated vesiec as " the fundamental form of all organization, the meeting point between the inorganic and the organic," and as "the starting point of the fugal progress of every higher indi vidual in creation, both animal and vegetable." Founding on instances of the produc tion of the proximate principles of which organic substances are composed in the labora tory of the chemist, he goes on to say that "an operation which would produce in these the nucleated vesicle is all that is wanting effectually to bridge over the space between the inorganic and the organic;" and that " it does not seem, afterall, a very immoderate hypothesis, that a chenoico-eleofric operation, by which germinal vesicles were produced, was the first phenomenon in organic creation, and that the second was an advance e these through a succession of higher grades, and a 'variety of modifleations in accordance with laws of the same absolute nature as those by which the Almighty rules the physical department of nature." He regards the idea of species or specific distinction, therefore, "as seemly applicable to certain appearances presented, perhaps transiently, to our notice." He adduces instances of great changes of form and character known to take place in the lower departments of nature, both animal and vegetable, as givi ig probability to the supposition, that in a long succession of generations, great changes may take place also in the higher.