DISTINCTIVE CHARACTERISTICS. 31 



species : the idea is one of most uncertain limit, 

 and differs according to the views of each investi- 

 gator. We know of many species of such stability 

 that the earlier definition would seem to find its 

 application in their case ; but there are others 

 again whose variability, indefiniteness, and inde- 

 finability have baffled all attempts at a more 

 accurate limitation. 



Species of this kind, consisting of a mere suc- 

 cession of forms, are now met with principally 

 among the lower orders of animals ; for instance, 

 the Sponges consist almost entirely of such a 

 succession of forms which seem to merge one into 

 the other. The Mammalia, on the other hand, 

 have in the course of the later geological periods 

 settled down more quietly. The days when they 

 were undoubtedly much more varied in form than 

 now, and like the lower orders developed almost 

 entirely into varieties, are passed. In the grand 

 sifting process of thousands of years, numerous 

 forms have dropped off and vanished, and the 

 majority of the mammals of the present day might 

 lead one to assume the stability of species. 



The Mammals of to-day are sharply separated 

 from the other vertebrates by a series of pecu- 

 liarities in structure and development. Even the 



