326 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1916. 



organisms will necessarily explain the succession of their former 

 states. Evolution is an irreversible process and one which has 

 not progressed at a uniform rate. We must not, then, expect to 

 verify necessarily by the present organisms all the facts disclosed 

 by morphology. It follows in my opinion that morphological data 

 may force upon us indirectly certain conclusions even though we 

 should have no experimental proof of them in contemporary nature. 



Because of this very limitation which I have just pointed out, 

 much of the difficulty of the study of the mechanism of evolution 

 arises and to this may be attributed many of the profound dif- 

 ferences among naturalists on the subject of evolutionary mech- 

 anism. The second part of the course will be devoted to the ex- 

 amination and the criticism of the solutions that have been proposed. 



In a general way, the study of the mechanism of evolution is 

 that of the reciprocal influence of agents external to the organisms, 

 on the one hand, and of the living substance, properly speaking, 

 on the other hand. There are, then, if you wish, the external fac- 

 tors which together constitute the environment, and the internal 

 factors which are the specific properties of the organism. These 

 two elements are very unequally accessible to us. The environ- 

 ment is susceptible of being analyzed with precision, at least as 

 far as the present is concerned, and we can surmise it with enough 

 probability as to preceding periods. We know very much less about 

 living matter, and especially about the way in which its properties 

 may have varied in the course of time. Hence one meets with two 

 tendencies which have been encountered ever since the evolutionary 

 question arose and which are still very definite and very contra- 

 dictory in their effects on the general theories of evolution. One 

 of those attributes a large share to the external factors and attempts 

 to explain facts by physicochemical actions which are directly ac- 

 cessible. The other sees in internal factors, in the intrinsic prop- 

 erties of the organism itself, preponderant if not exclusive agents. 



The first tendency attracts us more because it gives a larger share 

 to analysis ; that is to say, to the truly scientific method. The second 

 flatters our ignorance with fallacious verbal explanations. It is 

 open to the objections brought against vitalist conceptions; and 

 when, as is the case of certain old and new theories, we come to 

 restrict the effective role to internal factors alone, we may ask 

 ourselves whether there is a really essential difference between con- 

 ceptions of this nature and creationist ideas ; between declaring that 

 species have been created successively and arbitrarily by an arbi- 

 trary sovereign will, without the external world having influenced 

 their structure, or maintaining that organic forms succeed one an- 

 other, derived, to be sure, one from another but following a suc- 

 cession that is really determined in advance and independent of 



