672 ZOOLOGY. 



vidual and of the members of the class to which it belongs %. 

 the parallelism between the formation and differentiation 

 of the land-masses of the globe and the successive extinc- 

 tions and creations of plants and animals — all these facts, 

 notwithstanding the imperfections of the geological record,, 

 and the fact that many of the older forms of animals were 

 nearly as much specialized as those now living ; tend strongly 

 to prove that, on the whole, the world as it now exists has 

 been the result of progressive development, one form com- 

 ing genetically from another ; the animal and plant worlds 

 constituting two systems of blood relations, rather than sets 

 of independent creations. 



When to more special studies of those species which live- 

 in extraordinary environments, such as cave-animals, para- 

 sitic animals, brine-inhabiting animals, Alpine forms and 

 certain deep-sea species, we add the study of rudimentary 

 organs in adult animals, of temporary, deciduous organs in 

 Toung or larval animals ; when we compare the metamor' 

 phosesof some species congeneric with others which underg<> 

 no transformations ; when we study the delicate balance in 

 nature as observed in the geographical distribution of ani- 

 mals ; the harmony in nature between species and their en- 

 vironment ; protective coloration and resemblance in form, 

 the relations between carnivorous and herbivorous creatures, 

 the struggle for existence between animals, we are forced 

 to acknowledge that the operations of nature, as a whole, 

 tend, on the one hand, to the origination of new forms 

 and the preservation of those which are useful, or, in other 

 words, are in harmony with their surroundings ; and, on the 

 other hand, to the destruction of those which are incapaci- 

 tated by changes in their environment for existence in what 

 has been and now is a constantly changing, progressive 

 world. 



Again, reasoning by induction, as an actual fact we know 

 that species vary ; that hardly any two experts agree exactly 

 as to the limitation of species ;* that varieties tend to break 



* As one of many examples, we may cite the fact that fifty-nine nom- 

 inal species of the squirrels have been described as inhabiting tropical 

 America, but lately the number has been reduced to twelve. 



