PRINCIPLES OF VERTEBRATE MORPHOLOGY 25 



related species may be made to differ markedly from one another by 

 csenogenetic adaptive changes, as in the case of some of the frogs that 

 have adopted the habit of laying the eggs out of water. In these 

 cases the ontogenies are much more widely divergent than one would 

 expect in forms so closely related. So the statement of Lillie that 

 "ontogenies resemble one another in proportion to the nearness of 

 relationship," might be emended by adding the clause: — unless the 

 ontogenies have been secondarily disturbed by adaptive ccenogenetic modi- 

 fications. In aU of our attempts to make use of ontogenies as evi- 

 dences of phylogenetic relationships we must be on our guard against 

 the various pitfalls that have been discussed, and must not fail to 

 recognize that in phylogetic research palaeontology is a more reliable 

 guide than is embryology. 



The Geologic Aspects of Vertebrate Phylogeny 



In order to appreciate the historical side of vertebrate evolution 

 it is necessary to know something about the geologic succession of the 

 various classes. It is estimated that at least 25,000,000 years have 

 elapsed (some authors estimate ten times this) since the deposition 

 of the strata in which we find the earliest vertebrate fossils. Very 

 likely at least 5,000,000 years of evolutionary progress had been 

 made since the first chordate ancestors of these early armored verte- 

 brates had made their appearance. The accompanying chart (Fig. 5) 

 shows in compact form the great eras of geologic time and the number 

 of years that each is believed to have occupied. Another excellent 

 chart by Osbom (Fig. 6) indicates the characteristic vertebrates of the 

 various great geologic ages and the successive geologic appearance, 

 adaptive radiations and diminutions of the five vertebrate classes. 

 It will be noted that all of the classes arise in the Palaeozoic, that 

 each higher class arises, not late in the developmental history of the 

 ancestral class, but very near its base, when the earlier race was young. 

 Thus the Amphibia are shown coming off from fishes of the De- 

 vonian; the reptiles from the Lower Carboniferous Amphibia before 

 the Amphibia themselves had made any progressive adaptive radia- 

 tion; while both birds and manunals came off near the base of the 

 reptilian trunk, the birds probably somewhat later than mammals. 

 In addition, it will be noted that the first vertebrate remains occur 

 far down in the Palaeozoic, in Ordovician times, which were char- 

 acterized primarily by invertebrate forms. The student will find it 



