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SCIENCE 



[N. S. Vol. XLIII. No. 1113 



organisms, it is more than doubtful in 

 others, particularly in all those which re- 

 produce asexually, that is to say, many 

 large groups of animals like the Ccelente- 

 rata, the Bryozoa, the Tunicata, and many 

 plants. This has more than the force of an 

 exception, it is a general principle of the 

 life of species. One can not then say that 

 the conception of Weismann carries full 

 conviction. But this conception exercised 

 a tyrannical influence upon the minds of 

 contemporaneous biologists and it is ex- 

 clusively through it that most of them look 

 at the facts. 



Weismann, besides, exercised a consider- 

 able influence by championing a theory of 

 heredity based at the start on the preced- 

 ing ideas. This theory, built with un- 

 doubted ingenuity, and adapted to the 

 knowledge gained from the study of cell 

 division, turns out on the other hand to 

 agree with the recent works on heredity. 



Lamarckism and Darwinism shared the 

 support of biologists up to the end of the 

 nineteenth century, discussion being in gen- 

 eral restricted to speculation. The con- 

 troversy begun in 1891 between Weismann 

 and Spencer, who represented the two ex- 

 tremes, gives an idea of the extent to which 

 one could go in this direction. 



The last twenty years constitute indis- 

 puitably a new period in the history of 

 transformism where the field of discussion 

 has been renewed and scientists have 

 sought to give it a much more positive and 

 experimental character. Two kinds of in- 

 vestigation have been developed in this di- 

 rection : on one hand the methodical study 

 of variations, and on the other that of 

 heredity and especially of hybridization. 

 These two categories overlap. 



Note that this new point of view is not, 

 properly speaking, a study of evolution. 

 According to it, variation and heredity in 

 themselves, under present conditions, are 



analyzed independently of all hypothetical 

 previous states of the organism. After- 

 wards the results obtained with the La- 

 marckian, Darwinian and other succeeding 

 theories will be confronted. 



The sum of these researches, which are 

 now in high favor, is a new and important 

 branch of biology, which has received the 

 name of genetics. It defines for us in par- 

 ticular the hitherto very vague notion of 

 heredity and seems certain to lead us to an 

 analysis of the properties of living sub- 

 stance somewhat comparable to that which 

 the atomic theory has afforded concerning 

 organic chemistry. We can not maintain 

 too strongly its great importance. As far 

 as the theory of evolution is concerned the 

 results obtained up to this time have been 

 rather disappointing. Taken together, the 

 newly discovered facts have had a more or 

 less destructive reverberation. In truth the 

 results obtained do not agree with any of 

 the general conceptions previously ad- 

 vanced and do not show us how evolution 

 may have come about. They have a much 

 greater tendency, if we look only to them, 

 to suggest the idea of the absolute stead- 

 fastness of the species. We must evidently 

 accept these facts such as they are. But 

 what is their significance ? On the one hand 

 they are still limited, on the other hand as 

 I have already indicated above, and as I 

 shall try to show in the following lectures, 

 the advances made by the study of heredity 

 in organisms, at the present time and under 

 the conditions in which we are placed, does 

 not permit us to accept ipso facto the doc- 

 trine of heredity for all past time and 

 under all circumstances. 



To use a comparison which has only the 

 force of a metaphor but which will make 

 my thought clear, the biologist who studies 

 heredity is very much like a mathematician 

 who is studying a very complex function 

 with the aid of partial differential equa- 



