28 HYATT ON THE TERTIARY SPECIES 



A general and confused conception exists that certain characteristics either appear 

 "in uterc", or tend to be inherited at earlier or later periods in the life of individual 

 descendants, and, then becoming fixed in the organizations, are transmissible at 

 corresponding stages, and also, that in some way, some of these characteristics become 

 fixed in embryo, and thereafter are invariable for that particular division of animals. 



The law of acceleration appears to me at present to show the manner in which 

 characteristics, which are perpetuated, finally either disappear or become fixed in 

 the young, or even in embryo. This conclusion may be followed out by any one who 

 will arrange a series of animals or their shells, according to their adult affinities and 

 their developmental characteristics. He will then see that adult characteristics which 

 are introduced in ancestral forms, tend to reappear at earlier and earlier stages, as he 

 travels along the series. 



Though it is perhaps impossible for us to trace any type back to its begirming, and thus 

 substantiate this law for the truly embryonic characteristics, the conclusion is inevitable 

 that if it is a true expression for the mode of inheritance of any series of animals, it was 

 probably also true for their ancestors. 



Why, then, the invariability of the embryonic form and characteristics, as among the 

 Ammonoids and Nautili ? The explanation appears to me to present no great difficulty. 

 All perpetuated characteristics when crowded into the younger stages, and tending to 

 appear at younger and younger periods, must either replace the original embryonic form 

 and characteristics, or be crowded out by the constant incoming of new characteristics, 

 which are continually being originated and tending like them to be inherited by the law 

 of acceleration. 



Embryonic characteristics are subject to great variations, under the action of corre- 

 sponding changes in the environment. Witness the different degrees to which parasitism 

 has encroached upon the type characteristics of the males of Cirripedia and of the Epizoa, 

 shortening the periods during which the young show their typical Crustacean forms 

 and characteristics, and the inference becomes almost irresistible, that Taenia has lost its 

 original type characteristics at all stages of development by the same process. 



In what other way can we describe this as taking place, except by the law of 

 acceleration, by the earlier and earlier inheritance in successive generations of differential 

 characteristics first introduced in the adult stages of their ancestors by the influence of 

 the parasitic environment ? 



It is evident that suitability to the parasitic mode of life determined the selection of 

 these crustacean forms, otherwise we cannot understand their being found in such 

 habitats, or the pliability of their organization, or the fact that their young are 

 locomotive, and seek out the hosts in which we find them when full grown. 



In the same way we cannot understand the disappearance of perpetuated characteristics 

 in the young except on the hypothesis that they have become useless and are absorbed to 

 make room for the new ones which are inherited from later ancestors. They are met on 

 the one side by the embryonic type characteristics, which are the last to give way even in 

 the parasites, and on the other by the action of the law of acceleration, and they must 

 disappear or become embryonic. But room for them in the embryonic stages does not 

 appear to be found, except perhaps to a very limited degree. This we know from the 



