94 GENERAL EVOLUTION. 



rated ; the remaining bulbus is divided by a long valve or incom- 

 plete septum, tracing the division of the aorta- roots. In the ser- 

 pent (Rathke) this division is so accelerated as to appear 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 accel- 

 erated so as to appear long before the first named. Hence, in the 

 septa in the serpent, the singular anomaly seems to present of 

 the mammal passing through the Batrachian stage, while the ser- 

 pent, a nearer relative, does not.* If, however, we 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 parallelism," which is observed in comparing the develoj?- 

 mental histories of all tjrpes not most closely allied. It has not, 

 according to our theory, ahvays been a divergence, but was at a 

 prior epoch in each case a relation of "exact parallelism," the 

 lower type a repressed higher ; the former 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 ex- 

 act parallelism has always been a temporary relation, and one 

 shifting over the face of the system. 



III. OF HIGHER GROUPS. 



First. Comparison of the contemporary. 



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 broached naturally comes up : Has such trans- 

 formation of types, generic or higher, tahen place in any degree 

 siinuUaneously, throughout a great number of species f An af- 

 firmative answer to such a proposition is absolutely necessary to 

 its acceptance as expressing the phenomena exhibited by geolog- 

 ical succession of types. Let us try to answer the question put in 

 a closer form. Have the same species been transferred from one 



* This is the way indeed in wWch it is stated by Rathke, " Entwickelungsge- 

 schichte der Natter," p 164. 



