368 GEOLOGICAL BIOLOGY. 



critical points, with long periods of almost absolute cessation 

 of progress; and suggest that the part played by what is called 

 natural selection has determined rather the particular indi- 

 viduals and the place and time for advance steps, than, either 

 the direction of the steps themselves, or the relative value of 

 the particular modifications in relation to continuation of the 

 race, which have taken place. 



The study of the actual facts of the geological history of 

 organisms points unmistakably to a course of evolution by 

 descent, in which the progress attained by each succeeding 

 form was a paramount condition of the origin of the next 

 member of the race. 



Objection may be taken to an argument based on so few 

 examples. I think the force of this objection will be lessened 

 when we bear in mind that the examples were selected pri- 

 marily because of their fitness to testify upon the points ia 

 question, viz., the law of the history of organisms, the nature, 

 the rate, and the order of modification of form, which organ- 

 isms actually undergo in producing that divergence of specific 

 forms observed at any particular stage of the history. 



It may be said that the particular kinds of animals select- 

 ed do not fairly represent the total life of the world. To this 

 objection the reply may be made that a full quota of diversity 

 of specific forms has been attained by the races examined, 

 and the chief question before us is. How has that diversity 

 arisen? 



If the facts we have examined do not support the hypoth- 

 esis that the chief factor in organic evolution is either external 

 envnonnient or natural selection, it is not on account of any 

 lack of fitness to testify on this point, if it were true. 



The facts examined — and we believe that fuller examina- 

 tion of other statistics, both fossil and recent, will support 

 the same conclusion — show that evolution is rather an intrinsic 

 law of all organisms, and is to be discovered in the phenomena 

 of variation, which appear to be constantly active, rather than 

 in any accidental operations dependent upon the conditions 

 of external environment. 



The emphasis is placed upon the intrinsic rather than the 

 extrinsic factors of evolution, as the actual determinants of 



