PLASTICITY AND PERMANENCY OF CHARACTERS. 321 



die out in course of time is illustrated by thousands of species 

 which are represented abundantly in the rocks of some par- 

 ticular period, but thereafter are never seen again. Varia- 

 bility in ontogenesis is a necessity of living at all. The organ 

 which in its minutest characters has ceased to change, has 

 ceased to live ; and if we extend this generalization to the law 

 of phylogeny, we might expect to find, not a uniform, contin 

 uous evolution along all lines, but pulsations, so to speak, in 

 the activity of phylogenetic evolution of organisms along each 

 line. Taken as a whole, doubtless there is a gradual read- 

 justing of parts ; but each part is temporary, and is displaced 

 by another. So long as great flexibility of any particular 

 character, or set of characters, prevails, there will be rapid 

 appearance of new forms ; but after their initial appearance, 

 the repeating of the characters by natural generation will tend 

 to their fixation, and with the limitation of adjustability to 

 environment there will result death upon the slightest mal- 

 adjustment ; thus, as the variability of the species becomes 

 more and more narrow, the conditions under which it can 

 thrive become more and more restricted, and the final result 

 must be extinction. 



Whenever the action of heredity becomes restricted that 

 is, when sterility limits the range of variation within which 

 generation is possible this condition of fertility must work 

 toward the final extinction of the race. Thus, according to 

 this theory, if a species be found breeding perfectly true, we 

 can conceive it to have reached the end of its life-period, and 

 likely soon to become extinct. The theory in this respect 

 can be tested by the facts ; and although statistics as to the 

 actual fact on this particular point are wanting, it has been 

 frequently noticed in fossil species which have been care- 

 fully observed by the author that it is a conspicuous law, that 

 in respect to those characters which serve as distinctive marks 

 of species, there is greater general variability in the early 

 stages of the life of the genus. than in the later stages. The 

 following fact is an expression of the same law, viz. : the spe- 

 cies occurring at the early stage of a genus are generally more 

 difficult to separate, and there are more intermediate links 

 among earlier than among later species of a genus. After 



