1870.] 31 [Hyatt. 



mens which did not develop beyond periods corresponding to the 

 younger stages of lower species appears to be very probable. 



Quite a strong confirmation of this tendency of Birchii to have 

 dwarfish descendants is to be found in its own series, if we may so 

 call the only species which succeeds it and inherits all of its peculiar 

 characteristics. Microderoceras Hebertii Opp. * of the middle Lias is 

 precisely similar to M. Birchii in all its characteristics, except the 

 smaller size of the spines and the shorter diameter of the full grown 

 shell. The superior lateral lobes are not invariably equally divided 

 by a median minor cell, as in D'Orbigny's figure of this species, but 

 sometimes are unequally divided, this cell being thrown to one side 

 as m Hebertii. We know that Microceras Hebertii is very much 

 smaller than Microderoceras Birchii, because the shell enters upon the 

 old age or senile period of growth before the latter has attained its 

 fullest adult condition. 2 Ti^e whorls themselves do not differ in size, 

 so that the shell compares with Birchii in the same manner that bife- 

 rum or Deroceras Dudressieri compares with it, and in the same 

 manner that planicosta and D. confusum or laticosta compare with 

 these two ; they are as large as the young of the species which they 

 resemble in many cases in their whorls, the only difference being that 

 they do not have as many whorls, or attempt to develop the septa 

 beyond a certain youthful or immature condition. They may be said 

 to be arrested in development so far as size is concerned, and retro- 

 gressive in development when the reversionary characteristics are 

 considered. 



Darwin's close and exhaustive work upon the reversionary charac- 

 teristics of domesticated breeds is, to a certain extent, unsatisfactory, 

 since, while it points to a probable ancestor, it cannot, from the na- 

 ture of existing animals, show the preexisting steps by which the 

 change has been accomplished. The element of time, also, is com- 

 paratively short, and the whole evidence is necessarily hypothetical. 



In the cases given above, however, it will be noticed that while 

 the facts are not so numerous and conclusive as in the great pigeon 

 argument, they possess the additional confirmation derived from the 

 consideration of the manner of their introduction and their serial 

 succession in geological time. 



1 This is the Amm. brevispina B'Orbigny (not Sowerby). 



2 It should have been mentioned that D. Dudressieri begins its old age period on 

 the eighth whorl, while still very much smaller than the adult M. Birchii. • 



