WHAT ABE SPECIES? 415 



the course of their development, run through a series of changes of 

 the same order as those which are postulated by the evolution-theory 

 for life in time. 



Again, the facts of geographical distribution, as now known, are 

 absolutely incompatible with the hypothesis that existing animals and 

 plants have migrated from a common centre, whether Mount Ararat 

 or any other ; and, by demonstrating the similarity of the existing 

 fauna and flora of any locality to that which inhabited the same area 

 in the immediately precedent epoch, have furnished a strong argument 

 in favor of the modifiability of species. Thus, it is not too much to 

 say that the facts of biology known at the present day are all con- 

 sistent with and in favor of the view of species entertained by La- 

 marck, while they are unfavorable to, if not incompatible with, those 

 advocated by Cuvier; and that, even if no suggestion has been 

 offered, or could be offered, as to the causes which have led to the 

 gradual evolution of species, the hypothesis that they have arisen by 

 such a process of evolution would be the only one which would have 

 any scientific foundation. 



The great service which has been rendered to science by Mr. Dar- 

 win, in the " Origin of Species " is that, in the first place, he has mar- 

 shaled the ascertained facts of biology in such a manner as to render 

 this conclusion irresistible ; and, secondly, that he has proved the fol- 

 lowing proposition : Given, the existence of living matter endowed 

 with variability, the interaction of variation with the conditions of ex- 

 istence must tend to give rise to a differentialism of the living matter 

 into forms having the same moi'phological relations as are exhibited 

 by the varieties and species which actually exist in Nature. 



"What is needed for the completion of the theory of the origin of 

 species is,- first, definite proof that selective breeding is competent to 

 convert permanent races into physiologically distinct species ; and, 

 secondly, the elucidation of the nature of variability. It is conceiv- 

 able that both the tendency to vary and the directions in which that 

 tendency takes effect are determined by the molecular constitution of 

 a living body, in which case the operation of the changes of external 

 conditions will be indirect, and, so to speak, permissive. It is con- 

 ceivable, on the other hand, that the tendency to vary is both origi- 

 nated and directed by the influence of external conditions, while it is 

 also conceivable that both variation and the direction which variation 

 takes are partly determined by intrinsic and partly by extrinsic con- 

 ditions. The American Cyclopaedia. 



