442 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



world ? To answer this question, let us look a little into the past of 

 our planet and raise, as much as nature will permit us, the curtain 

 that shrouds its past history. 



In looking into the past and present of the animal world of the 

 terra firma we perceive one fact — two types of animals are striving 

 for dominance. These two types are the vertebrates and the arthro- 

 pods. True, parts of both types (fishes and crustaceans) remained in 

 the water, in their native medium, but in the present case they will not 

 interest us. But in the evolution of insects (as well as the other 

 classes of terrestrial arthropods) on the one hand and terrestrial ver- 

 tebrates on the other we see a striking contrast; we have before us 

 one of those characteristic instances full of deep significance, where 

 nature, in aiming for the same goal, proceeds and attains it by means 

 of two opposite routes. 



If this goal is assumed to be the protection of the species in the 

 struggle for existence, what are the paths along which the vertebrates 

 and the insects traveled ? These roads are hidden from us in the deep 

 mystery of the past ages, and only scant, fragmentary, and scattered 

 data for study are given us by the paleontological discoveries. 



The first impression we get from these data is that in early geo- 

 logical epochs the vertebrate world was incomparably larger, more 

 massive than in the contemporary; that the type of vertebrates ap- 

 pears to be degenerating, growing smaller. 



Indeed, a whole series of gigantic forms which previously popu- 

 lated the earth has completely disappeared from its face: all the 50- 

 feet long Brontosauri, Mastodonsauri, elephant-like Dinothers, Mas- 

 todons, and many, many others died out, and the vast majority of con- 

 temporary vertebrates can not compare with them in size of body. 

 However, on closer study we will note a different aspect. We will see 

 that none of these vertebrate giants are the ancestors of the present 

 forms. On the contrary, these are all forms which are always ex- 

 tremely and one sidedly specialized, adapted to definite and, doubtless, 

 limited conditions of existence. And what is no less important, to 

 which I wish to call special attention, is the fact that these gigantic 

 forms appear as if they always conclude a series of links of a chain of 

 successive forms, at which it suddenly breaks. These chains usually 

 begin with small forms, with primitive peculiarities of structure and 

 only as specialized characters accumulate does the size of the animals 

 grow until it attains gigantic proportions and extreme specialization, 

 and then the power of adaptation to changed conditions of existence 

 seems to disappear and the entire chain of forms closes its earthly 

 existence. For illustration I will present a few examples. 



The class Amphibia at first appears in the lower carboniferous de- 

 posits as small salamander-like forms, Branchiosaurus, which belong 

 to the subclass StegocephalL But in the triassic we come across such 



