204 GENERAL EVOLUTION. 



ure, as a tube, a phalange, a digit, a limb or an arch ; an ocellus 

 or a tooth. For instance, in the genus Amhlystoma, one section 

 possesses four phalanges on the longest digit ; another section 

 exhibits but three. In the species A, mavortium, some individuals 

 have the small number of phalanges, but the majority possess the 

 larger number. As all are of common parentage, a whole phalange 

 has been lost or added. The explanation of this phenomenon is 

 essential to the comprehension of the origin of type structures. 



* In plants, growth-nutrition continues throughout life, but in 

 the higher plants it is more active during the earlier years in peren- 

 nial species, addition to size becoming less and less marked with 

 increasing age. Grade-nutrition also persists throughout life, but 

 is chiefly active during a short period only of every year, or during 

 flowering and fruiting. Not only in the production of the repro- 

 ductive organs, but also in the yearly additions to other tyj^ical 

 parts of the plant, grade-nutrition is active. 



** In animals, growth-nutrition is more active in the early 

 stages of life, but is continued throughout in the lower divisions ; 

 in the highest, it is also continued throughout life, but there is a 

 greater contrast between its results during youth, when nearly the 

 whole size is attained, and during age, where the additions are 

 much less. 



Grade-nutrition is, on the other hand, entirely confined to in- 

 fancy and youth, except in those low animals which produce their 

 reproductive organs periodically (some Fntozoa, etc.), where it may 

 be said to be in nearly the same condition as in plants. 



y. While the amount of simple growth-force, represented in 

 adult living animals, has varied very irregularly throughout the 

 animal kingdom, there being large and small in every division, it 

 would seem to have accumulated, on the whole, with the rising 

 scale of animal types. Thus the lower or Protozoa are the small- 

 est ; Radiates are next in size ; Molluscs and Articulates reach 

 nearly the same maximum, which exceeds that of the Radiates, 

 and falls far below that of the Vertebrates. Among the last the 

 Mammalia have attained as large if not larger size than any of the 

 other orders (e. g., Cetaced). This is, however, not necessary to 

 the history of evolution. 



That an increased amount of grade growth-force has been con- 

 stantly rendered potential during the advance of time is clear, if 

 the preceding inferences be true. It is also evident that some in- 

 dividuals have accumulated it more rapidly than others. If all 



