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 aniinals, of temporary, deciduous organs in 
young or larval animals; when we compare the metamor- 
phoses of some species congeneric with others which undergo 
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 changesin 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. 
