NATURAL SCIENCES OF PHILADELPHIA. 279 



velopment in the individuals of the species of the extreme type, such stimulus 

 being more and more vigorous in the individuals of the types as we advance 

 towards the same, or by a reversed impulse of development, where the ex- 

 treme is characterized by absence or " mutilation " of characters. 



IX. And that as the character of tiie genus at the period of reproduction of 

 its species, is that which is perpetuated; 



X. So the character of the genus has been first inferior, then protean, and 

 then advanced, as the metamorphosis has been by a retrograde movement in 

 time, posterior to, at, or anterior to the period of reproduction. 



XI. That it therefore results that there is one primary structural type in- 

 volved in such a series of species, which is made to present at any given 

 period in its Geologic history that appearance of succession of genera or- 

 dained by Creative Power. 



/. On the origin of inexact parallel ism. 



The hypothesis can only be demonstrated ia case of exact parallelism. If 

 proven in these, it readily accounts for the cases of inexact parallelism, which 

 are of course in any single period vastly in the majority. First take the case 

 of simple inexact parallelism-. A series of individuals of the genus Didocus un- 

 dergo the metamorphosis of the cranial structure earlier and earlier in life, 

 commencing by completing the ossification of the perichondrium of the fronto- 

 parietal region in full age, until at last it becomes completed as earlj^ a^ the 

 period of reproduction. Heretofore the adult offspring have appeared during 

 a long period, invariably characterized by the larval cranium, but like now 

 producing like, this development springs into new power, and the offspring 

 ossify the cranial bones, tar earlier than their immediate predecessors ; in a 

 word, the genus Pelobates has been created ! At this state of progress Di- 

 docus is an undeveloped Pelobates. 



Let us, however, suppose the. "acceleration" of development of the crania! 

 bones still to progress. The character appears now soon after the ordinary 

 metamorphosis has been passed, and now a little before. The identity of Di- 

 docus with the undeveloped Pelobates is thereupon lost ! 



So may have been the relations between Pelobates and Cultripes. Peloba- 

 tes was probably onceddentical with the undeveloped Cultripes, but the same 

 acceleration has concentrated the characters more rapidly than the other 

 larval stages, leaving Pelobates behind. 



This I conceive to be the explanation of tliis relation : when the parallel- 

 ism is inexact by two steps, as in Spea to Didocus, by the obliterated ear and 

 ossified xiphisternum. The continued concentration of characters has been 

 carried to earlier stages till the identity exists in the adult state of neither one, 

 but at a period of larval life of both, shorth' preceding the adult period of the 

 lower. The- relations between the Amblystomid;e and Plethodontidie, which 

 I have elsewh.ere * pointed out, have probably had their origin in this way. 



If we attempt to prove the identity of the modern mammalian fa?tal circu- 

 lation with that of the modern adult fish, we may find nearly an exact parallel 

 in this respect, as it is the basis of class distinction ; but in other respects the 

 identity will not exist, rendering the parallel inexact or remote The structure 

 of the origins of the aorta is at one time identical with that of the shark, 

 with OBe exception — in the former but four aorta-bows appear together ; in the 

 latter five. In the former the first disappears as the fifth comes into being. 

 This is simply a continuation v>f acceleration. The first generalized representa- 

 tive of the iMammalia lost the first aorta-bow towards the latter part of its 

 growth, and became the next genus in advance of the selachian. The fact 

 that these bows do not appear exactly simultaneously, but rather successively, 

 renders it necessary that in a regularly shortening period of possession of 



* Jour. Ac. Nat. Scl, Phil., 1866, 100. 



1868,] 



