Apkil 21, 1916] 



SCIENCE 



651 



longer be any question of systematically 

 regarding individual development as a 

 repetition of the history of the stock. This 

 conclusion results from the very progress 

 made under the inspiration received from 

 this imaginary law, the law of biogenesis. 



The first part of the course will be de- 

 voted then to the consideration of the gen- 

 eral data which morphology furnishes to- 

 ward the support of the idea of evolution. 

 Thus we shall see what conception com- 

 parative anatomy, embryology and pale- 

 ontology affords us of the way in which evo- 

 lution is brought about, and within what 

 limits we may hope to reconstruct it. Evo- 

 lution is essentially a process which belongs 

 to the past and even to a past extraordi- 

 narily distant. It is a reasonable supposi- 

 tion that evolution is going on to-day, but 

 let us remember that nothing authorizes us 

 to believe that what we may observe in the 

 present epoch about organisms will neces- 

 sarily explain the succession of their former 

 states. Evolution is an irreversible process 

 and one which has not progressed at a uni- 

 form 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 contempor- 

 ary nature. 



Because of this very limitation which I 

 have just pointed out, much of the difS- 

 eulty of the study of the mechanism of evo- 

 lution arises and to this may be attributed 

 many of the profound differences among 

 naturalists on the subject of evolutionary 

 mechanism. The second part of the course 

 will be devoted to the examination and the 

 criticism of the solutions that have been 

 proposed. 



In a general way, the study of the mech- 

 anism of evolution is that of the reciprocal 



influence of agents external to the organ- 

 isms, on the one hand, and of the living 

 substance, properly speaking, on the other 

 hand. There are then, if you wish, the ex- 

 ternal factors which together constitute 

 the environment, and the internal factors 

 which are the specific properties of the or- 

 ganism. These two elements are very un- 

 equally accessible to us. The environment 

 is susceptible of being analyzed with pre- 

 cision, at least as far as the present is con- 

 cerned, 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 tend- 

 encies which have been encountered ever 

 since the evolutionary question arose and 

 which are still very definite and very con- 

 tradictory in their effects on the general 

 theories of evolution. One of those attri- 

 butes a large share to the external factors 

 and attempts to explain facts by physico- 

 chemical actions which are directly acces- 

 sible. The other sees in internal factors, 

 in the intrinsic properties of the organism 

 itself, preponderant if not exclusive agents. 

 The first tendency attracts us more be- 

 cause it gives a larger share to analysis, 

 that is to say to the truly scientific method. 

 The second flatters our ignorance with fal- 

 lacious verbal explanations. It is open to 

 the objections brought against vitalist con- 

 ceptions ; and when, as is the ease 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 concep- 

 tions of this nature and creationist ideas; 

 between declaring that species have been 

 created successively and arbitrarily by an 

 arbitrary sovereign will, without the exter- 

 nal world having influenced their struc- 

 ture, or maintaining that organic forms 



