142 GENERAL EVOLUTION. 



reproduction. The acceleration in the assumption of a character, 

 progressing more rapidly than the same in another character, must 

 soon produce, in a type whose stages were once the exact parallel 

 of a permanent lower form, the condition of inexact parallelism. 

 As all the more comprehensive groups present this relation to each 

 other, we are compelled to believe that acceleration has been the 

 principle of their successive evolution during the long ages of ge- 

 ologic time. 



Each type has, however, its day of supremacy and perfection 

 of organism, and a retrogression in these respects has succeeded. 

 This has no doubt followed a law the reverse of acceleration, which 

 has been called retardation. By the increasing slowness of the 

 growth of the individuals of a genus, and later and later assump- 

 tion of the characters of the latter, they would be successively lost. 



To what power shall we ascribe this acceleration, by which the 

 first beginnings of structure have accumulated to themselves 

 through the long geologic ages complication and power, till from 

 the germ that was scarcely born into a sand-lance, a human being 

 climbed the complete scale, and stood easily the chief of the 

 whole ? 



In the cases of species, where some individuals develop farther 

 than others, we say the former possess more growth-force, or 

 "■ vigor," than the latter. We may therefore say that higher types 

 of structure possess more *' vigor" than the lower. This, how- 

 ever, we do not know to be true, nor can we readily find means to 

 demonstrate it. 



The food which is taken by an adult animal is either assimi- 

 lated, to be consumed in immediate activity of some kind, or 

 stored for future use, and the excess is re-jected from the body. 

 We have no reason to suppose that the same kind of material 

 could be made to subserve the production of force by any other 

 means than that furnished by a living animal organism. The ma- 

 terial from which this organism is constructed is derived first from 

 the parent, and afterward from the food, etc., assimilated by the 

 individual itself so long as growth continues. As it is the activity 

 of assimilation directed to a special end during this latter period 

 which we suppose to be increased in accelerated development, the 

 acceleration is evidently not brought about by increased facilities 

 for obtaining the means of life which the same individual possesses 

 as an adult. That it is not in consequence of such increased fa- 

 cilities possessed by its parents over those of the type preceding it 



