ON THE ORIGIN OF GENERA. 75 



tions of these species and genera are zoologically the same as 

 though this modification did not occur. Many such instances will 

 occur to many naturalists. It is not pretended that they are as 

 important as the presence of the allantois ; but they constitute a 

 character, no doubt, similar in kind, and entirely at the service of 

 the needs of the great system of morphological succession. The 

 same may be said of the vascular area of the Reptile. 



Lereboullet concludes his summary of the differences between 

 the Teleost and Reptile, up to the period of completion of the 

 heart, by saying, *^ It is easy to perceive that all these differences, 

 however imj)ortant they may appear, are constituted by the acces- 

 sory organs of the embryo, and do not modify the development of 

 the latter, which progresses in reality exactly as in the fishes." 

 He says the same previously, as to the relation of the same to the 

 bird and mammal. 



We have, then, in the embryos of the lower vertebrates at a 

 certain time in the history of each, an ''exact parallelism''^ or 

 identity with the embryonic condition of the type which pro- 

 gresses to the next degree beyond it, and of all the other types 

 which progress successively to more distant extremes. 



We have, however, so far, every reason to suppose that the 

 embryos of the other branches of animals never present an exact 

 parallelism with those of the Vertebrata.* 



The embryo of the fish and that of the reptile and mammal 

 may be said to be generically, if not specifically, identical up 

 to the point where preparation for the aerial respiration of the 

 latter appears. They each take different lines at this point. 

 The fish diverges from the course of the reptile, and proceeds 

 to a different goal ; the shark does the same, but proceeds a 

 shorter distance ; while the Dermopter scarcely leaves the point 

 of departure. No doubt, there have been types which never 

 left this point and tvliose plan of circulatory system is identical 

 with that of the embryo reptile and mammal. Such a type was 

 only generically different from the reptile or mammal ivhich had 

 only taken the succeeding step, provided other structures were 

 not superadded. 



By comparing the development of types of different classes in 



* At about the time this was written the important papers of Haeckel on his 

 admirable Gastraea theory were published, but had not reached the author. Haeckel 

 shows the approximate identity of all the types of embryonic development. (Note, 

 1886.) 



