280 PROCEEDINGS OF THE ACADEMY OF 



liausitory clmracters, one such, as the existence of the first aorta root, should 

 vanish before the appearance of a permanent, the fifth, in the more specialized 

 types, where acceleration reaches its maximum. This is indicated by the fact 

 tliat in the Batrachia, where the acceleration has not attained so high a degree, 

 the first and fifth aorta-bows coexist for some time, though the first and second 

 disappear before maturity. 



So also with the splitiing of the bulbus arteriosus. As in the Batrachia, the 

 jiulmonaiy (/ttc/iis cowmvnis only is to be separated, the remaining bulbus is 

 divided by a long valve or incomplete septum, tracing the division of the 

 aorta roots.- In the serpent (Rathke), this division is so accelerated as to ap- 

 ])ear at nearly the same time as the septum of the pulmonary duct. In the 

 mammal, on the other hand, while the division of the aorta root takes place 

 as soon as in the last, the pulmonary septum is accelerated so as to appear 

 long before the first named. Heiice in the septa in the serpent, the singular 

 anonnily seems to present, of the mammal passing through the Batrachinn 

 stage while the serpent, a nearer relative, does not.* If, however, M-e take the 

 less typical serpent, we will find the aortic septum to appear a little later, thus 

 giving the Batrachian type, and if we reverse the order of time, so that the 

 succession becomes one of retardations, we will find the same known ratio 

 will bring us to an identity under all circumstances. 



This then is the explanation of the divergence and want of "exact parallel- 

 ism " which is observed in comparing the developmental histories of all types 

 rwt niosl close/'!/ allied. It has not, according to our theory, always been a diver- 

 gence, but was at a prior epoch in ea( h case a relation of" exact parallelism," 

 the lower type a rej)ressed higher; the foimer identical with one of the stages 

 of the latter. But the process which has produced this relation, continued, 

 has of necessity destroyed it, so that the exact parallelism has always been a 

 temporary relation, and one shifting over the face of the system. 



III. 0/ liigher groups. 

 First ; comparison of the coternporary. 



Having now admitted a developmental succession of genera, and second, that 

 this has progressed more rapidly at certain times in the earth's history than 

 any modification of specific forms, the hypothesis already broaclied nat- 

 urally comes up. lias such transformalion of types, generic or higher, takm 

 place in any degree simultaneously, throughout a great number of species? An 

 affirmative answer to such a proposition is absolutely necessary to its accep- 

 tance as expressing the phenomena exhibited by geological succession of types. 

 1-et us try to answer the question put in a closer form. Have the same 

 species been transferred from one geologic epoch to another by a change of 

 generic form ; and has not the genus been transferred from one epoch to 

 another under change of ordinal type ? and as a consequence the same species V 



As a reply, I propose to render the affirmative of the first of these questions 

 highly probable. 



Pahvontology only will be able to answer this question conclusively, though 

 as we have abundant evidence that the relations of species to genera and other 

 higher groups were the same then as now, we may look to the present status 

 as furnishing important evidence on the subject. We are turned at once to 

 the probable history of development in the separate zoological areas of the 

 earth's surtace. The question may l)e asked, Are the present zoological regions 

 on an equal plane as to the geologic relations of their fauna^, or are they re- 

 lated as the different subdivisions of a geologic period in time? 



I have on a former occasion asserted that the latter of these propositions 

 was trne.f 



*This i.« tlie way indeed in which it is stated by Ratlike, Entwickelungsgesehichte der 

 Natter p. ]i)4. 

 + Ou Areiferous Anura, Jonrn. Ac. Nat. Sci., 1800, 108. 



[Oct. 



