356 PROCEEDINGS OF THE ACADEMY OF [May, 



From the above study the writer is led to beUeve that there are two 

 modes in which evohitional features may develop. First, they may 

 develop slowly and evenly in the growth of the individual (ontogeny), 

 and in the evolution of the race in geologic time (palseontogeny ) . 

 Such forms are stable and persist for long periods of time. Second, 

 the same evolutional features may develop in the same order, but more 

 rapidly; one feature following another in quick succession in the on- 

 togeny and paltEontogeny. Such forms are apt to be short lived, soon 

 becoming senile and dying out. Where the rate of development of 

 evolutional features is rapid, we find that some features are slurred 

 over; while other features may be more exaggerated than the corre- 

 sponding features in the forms whose rate of development of characters 

 is slow. Of course acceleration, or the throwing back of characters into 

 the ontogeny, is a natural result of an increased evolutional rate. 



Further, we cannot escape the conclusion that there are times in the 

 evolution of a group of organisms (palseontogeny) which might be 

 termed periods of pliancy. In these the simple, generalized members 

 of the main ancestral stem can be easily moulded. At such periods of 

 pliancy we may expect to find, first, forms which follow out the suc- 

 cession of developmental features, as in the main stock, but at a more 

 rapid rate, becoming senile and extinct; and second, forms which 

 acquire characters of a more or less permanent nature, and which do 

 not go through the same succession of developmental features which 

 is found in the main ancestral stock. These latter forms make new 

 stocks, or, in other words, they go to form new species, genera, etc. 



Therefore, at every period of pliancy in a stock we have, on the one 

 hand, new species and, on the other, senile offshoots. In the forms 

 above considered, that is, the Fulgurs, the numerous senile offshoots 

 far outnumber the few but persistent stable forms. The following 

 diagram may be used : 



