274 GEOLOGICAL BIOLOGY. 



to the conditions of environment can be tested. It is reason- 

 able to expect, therefore, that all modifications of organic 

 structure, which imply strictly intrinsic differentiation of the 

 co-ordinated structure and function of the organism, were 

 evolved by processes vastly more rapid than those of the ex- 

 trinsic modification of structures already present in the race. 



We have seen how Brachiopods furnish us with the data 

 with which to trace the laws of the historical evolution of the 

 more important characters exhibited by any particular Brach- 

 iopod. These characters have fallen into natural divisions, 

 or groups of various rank, which are scientifically recognized 

 as class, ordinal, subordinal, etc., characters. We have seen 

 how the characters which we call subordinal, when they are 

 arranged in the order of their morphological affinities, present 

 a series of forms whose elaboration is as complete by the 

 beginning of the Upper Silurian as it was at any later time; 

 therefore we drew the conclusion that so far as the subordinal 

 characters and those of higher rank are concerned, the differ- 

 entiation expressed by these characters took place in the 

 lower half of the Paleozoic time. As far as the facts are in 

 evidence, we find that the characters of this kind were rapidly 

 introduced: rapidly in relation to the degree of differentia- 

 tion indicated by the characters, and rapidly in comparison 

 with the length of time they persist without apparent modi- 

 fication. As two ontogenetic forces are at work in the 

 growth of the individual, to which respectively we apply the 

 terms heredity and variability, so we recognize upon analysis 

 of the facts of the phylogeny two kinds of evolution : (I) a 

 progressive evolution which operates from within and is asso- 

 ciated with pre-existing conditions ; this is called intrinsic evo- 

 lution ; (II) another kind of evolution, observed to be more 

 intimately co-ordinate with external conditions, which may be 

 regarded as fundamentally a process of adjustment or adapta- 

 tion of the organism to its external environment ; and this is 

 extrinsic evolution. 



In the ontogenetic development of the individual there is 

 a rapid elaboration of those typical features of the organism 

 which express its class, ordinal, and subordinal characters, the 

 whole framework and plan of structure being elaborated 



