INTR OD UC TION. 9 



concentration of the adult characteristics in the young 

 of higher species and a consequent displacement of 

 other embryonic features which had themselves also 

 previously belonged to the adult period of still lower 

 forms." My own language is i 1 "That the presence, 

 rudimental condition, or absence of a given generic 

 character can be accounted for on the hypothesis of a 

 greater rapidity of development 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 im- 

 pulse 2 of development, where the extreme is charac- 

 terized by absence or ' mutilation ' of characters." The 

 phenomena of the aggregation of characters in pro- 

 gressive evolution, and the loss of characters in retro- 

 gressive evolution, were termed by me acceleration and 

 retardation in an essay published in i86g. 3 In these 

 papers by Professor Hyatt and myself is found the 

 first attempt to show by concrete examples of natural 

 taxonomy, that the variations that result in evolution 

 are not multifarious or promiscuous, but definite and 

 direct, contrary to the method which seeks no origin 

 for variations other than natural selection. In other 

 words, these publications constitute the first essays in 

 systematic evolution that appeared. By the discovery 

 of the paleontologic succession of modifications of the 

 articulations of the vertebrate, and especially mamma- 

 lian skeleton, I first furnished an actual demonstration 

 of the reality of the Lamarckian factor of use, or mo- 



1 Transactions American Philosophical Society, 1866, p. 398; reprinted in 

 The Origin of the Fittest, p. 92. 



2 The expression " reversed " is unfortunate, diminished being the proper 

 word to convey the meaning intended. 



3 The Origin of Genera, Philadelphia, 1869. 



