4 i2 THE POPULAR SCIENCE MONTHLY. 



Thus fertility, like parentage, has become a physiological character of 

 species ; and, though in the case of some domesticated animals, as 

 pigeons, the exti'eme forms are more different from one another than 

 many morphological species, yet they, apart from the historical evi- 

 dence of their parentage, are held to be members of the same species, 

 because they are all perfectly fertile one with another, and their off- 

 spring are also perfectly fertile. 



Thirdly, it is a matter of experience that, as a general rule, and 

 taking the whole cycle of forms through which a living being runs 

 into account, offspring and parent are so similar that they belong to 

 one and the same morphological species ; and it is further in evidence 

 that many species have endured for extremely long periods without 

 any notable difference being discernible between ancestor and de- 

 scendant. Moreover, in some cases, varieties are found to revert to 

 the character of the species from which they have proceeded. The 

 conclusion has been drawn that the character of species is physio- 

 logically fixed ; that is to say, that, however long the process of gen- 

 eration may be continued, the individuals either retain the identical 

 morphological character of the oldest ancestor, or, if they vary, the 

 varieties remain fertile with one another. 



Assuming that species have the physiological character thus enu- 

 merated, certain conclusions respecting the " origin of species " are 

 inevitable. It is clear that no existing species can have arisen by the 

 intercrossing of preexisting species, or by the variation of preexist- 

 ing species, but that every species must have existed from all eter- 

 nity, or have come into existence suddenly in its present form, which 

 is the objective fact denoted by what is termed creation. 



At the dawn of modern biology, a century ago, no scientific evi- 

 dence respecting the real history of life on the globe was extant, and, 

 for any proof that existed to the contrary, species might have been 

 of eternal duration. But philosophical speculation combined with 

 theological dogma not only to favor the contrary opinion, but to lead 

 the most philosophic naturalist of his day to embody the hypothesis 

 of creation in a definition of species. " Totidem numeramus species 

 quot in principio formae sunt creatse " (we reckon as many species as 

 there were forms created in the beginning) is the well-known formula 

 of Linnaeus. 



In practice Linnseus regarded species from a purely mythological 

 point of view ; in theory, he assumed the common ancestry and the 

 limited variability of species, though he was disposed to allow more 

 freedom in this direction than most of his successors. On the other 

 hand, he seems to have attached comparatively little weight to the 

 assumed sterility of hybrids, and to have held a sort of modified doc- 

 trine of evolution, supposing that existing species may have been 

 produced by the interbreeding of comparatively few primordial 

 forms. 









