Hyatt,] 382 [Juiie 7, 



cording to the law of acceleration, entirely omitted in the develop- 

 ment of the individuals of some of the completely senile or later 

 occurring species. The form of the embryo and the Nautiloid and 

 Goniatitic stages which were variable characteristics during the Silu- 

 rian and Devonian, have become more or less fixed and perma- 

 nent by constant inheritance, and are at this period in the existence 

 of the Ammonoids, either partially or entirely independent of the 

 action of the physical surroundings, occurring in the embryonic or 

 early stages of the development of the individual, however different 

 its habitat. I do not mean, of course, to assert that even the most 

 invariable of these hereditary characteristics did not arise primarily 

 as the direct product of physical causes, but simply to point out their 

 existing independence, after having become through continued he- 

 redity a permanent part of the growth tendencies of the group. The 

 proofs of this have been given in my paper on the " Embryology of 

 the Cephalopods," in which the gradual manner in which the char- 

 acteristics become less and less subject to variability in the embryo 

 is given in detail. 



The differences, then, or those characteristics distinguishing the 

 different series from each other when they first appear, must be 

 largely confined to the adult period in the existence of the individual 

 or to the later stages of the growth of the young, and this is a cor- 

 ollary of the proposition that the differences between the forms are 

 due to the direct action of different physical surroundings upon sim- 

 ilar organisms. For if the differences were thus produced we should 

 necessarily anticipate that they would make their first appearance, in 

 most cases at least, after the permanent and hereditary character- 

 istics had been fully developed. In common with Prof. Cope I have 

 repeatedly explained these and other related phenomena, by what 

 we have called the law of acceleration. [It is a universal law of 

 heredity, that previously elaborated, ancestral characteristics tend 

 to be inherited, if inherited at all, at earlier and earlier stages in 

 successive descendants, until they either finally disappear like the 

 Pettos-form in the young Sauzei, or become fixed and more or less 

 permanent in the embryo. 



Laying aside all of these, we can now turn our attention again to 

 strictly senile characteristics. These are the representative forms 

 which are produced in every series. That is to say, there is a certain 

 parallelism produced by the perpetual reappearance or genesis of sim- 

 ilar forms in distinct structural series, and as might be anticipated, 



