( 436 ) 



tion of these three kinds of polymorphism in the previons lines led us to the 

 conclusion that morphological differences of any kind and degree are not decisive 

 criteria as to specific distinctness ; the systcmatist actually sinks his species in 

 spite of distinguishing characters as soon as it is j)roved that the morphologically 

 different forms appear among the offspring of the same female. The most general 

 case of bodily difference which is not regarded as being specific is the difference 

 between males and females; notwithstanding the great dissimilarity which the 

 sexes so often exhibit, not only in the reproductive organs, but also in other 

 moi-phological characters, the systematist puts Diale and fem,ah together in one 

 species, and hence makes at once the concession that his term "species" is not a 

 pureh" morphological one, but that the higher criterion of the term is of a 

 physiological kind. 



Although moqjhological identitv means also specific identity, the inverse that 

 specific identity means morphological identity is not correct. The question, there- 

 fore, is now, which physiological divergence we will take as the real criterion of 

 specific distinctness. 



We have seen above that the line of ancestors of a given type can be divided 

 into a recent portion which is independent of the lines of ancestors of all other types, 

 and into a remote portion in which the lines of several tvpes are combined. The 

 specific difference which now keeps allied types separate was absent from them in 

 remote times ; what now is specifically different was formerly specifically identical. 

 Hence a definition of " species," i.e. a definition of what makes two types specifically 

 different, has to exclude any relation to the ancestral forms of the given t3'pes, but 

 has to take into consideration the contemporary types and their descendants. For 

 the sake of argument let us assume all types of animals and plants were monogamic. 

 so that every individual would produce offspring without copulation with anotTier 

 individual. The question as to the characters of the descendants would be twofold : 

 first, the descendants of a type (taken as a whole) become under certain conditions so 

 changed that the gap which separates them from the descendants of another type is 

 entirely filled up by intergradations, or that the descendants of both types entirely 

 fuse together in characters ; or, second, the sum of the descendants of each type 

 remains under all conditions separated b}' a morphological gap from all other sums 

 of descendants, whether the characters change or not. We have already referred to 

 the transplantation of plants and animals by which it has been proved that forms 

 which were unhe.'^itatingly considered specifically different became identical. The 

 divergency in the development of forms has in these cases been annihilated, and we 

 must conclude that similar divergencies will in nature also be annihilated when the 

 necessary conditions arise. If, therefore, all the various types in animated nature were 

 different only to that degree, it follows that under favourable circumstances all these 

 different forms would fuse together to one single type. Divergency would change 

 into convergency and identitj', and there would be no question as to species. As we, 

 however, observe that fusing together is restricted to the nearest allied types, and 

 that with the greater divergency of allied forms the possibility of fusing decreases 

 until the forms remain separate nnderany condition, the term "s[)ecies" as an expres.sion 

 of the divergency in nature must be an expression of that divergency which, though 

 starting from identity in the ancestral forms, will never again develop into identity. 



In the case of sexual propagation the question is more intricate in consequence 

 of the intercrossing whicli takes place between different types. The question as to 

 the lines of descendants is threefold : the line of descendants of a given tj'pe fuses 



