WEISMANN'S THEORY OF EEREDITY.* 



By George J. Romanes. 



The recently published translation of Professor Weismann's essays 

 on heredity and allied topics has aroused the interest of the general 

 public in the system of his biological ideas. But seeing that his 

 system, besides being somewhat elaborate in itself, is presented in a 

 series of disconnected essays, originally published at different times, it 

 is a matter of no small difficulty to gather from the present collection 

 of these essays a complete view of the system as a "whole. Therefore I 

 propose to give a brief sketch of his several theories, arranged in a 

 manner calculated to show their logical connection one with another. 

 And in order also to show the relation in which his resulting theory 

 of heredity stands to what has hitherto been the more usual way of 

 regarding the facts, I will begin by furnishing a similarl^^ brief sketch 

 of Mr. Darwin's theory upon the subject. It will be observed that 

 these two theories constitute the logical antipodes of explanatory 

 thought; and therefore it may be said, in a general way, that all other 

 modern theories of heredity — such as those of Spencer, Haeckel, Blsberg, 

 Galton, Naegeli, Brooks, Hertwig, and Vries — occupy positions more 

 or less intermediate between these two extremes. 



When closely analyzed, Mr. Darwin's theory — or " provisional hy- 

 jiothesis of pangenesis'''' — will be found to embody altogetiier seven 

 assumptions, viz: 



(1) That all the component cells of a multi-cellular organism throw 

 off inconceivably minute germs or ''gemmules, " which are then dis- 

 persed throughout the whole system. 



(2) That these gemmules, when so dispersed and supplied with 

 proper nutriment, multiply by self-division, and under suitable condi- 

 tions, are capable of developing into physiological cells like those from 

 which they were originally and severally derived. 



(3) That while still in this gemmular condition, these cell seeds have 

 for one another a mutual affinity, which leads to their being collected 

 from all parts of the system by the reproductive glands of the organism ; 

 and that, when so collected, they go to constitute tlie essential material 

 of jhe sexual elements, ova and spermatozoa being thus nothing more 



*Froin The Contcmporarij Review, May, IdDO, vol. i.vii, pp. (J86-G'.>9. 



fl. Mis. 12d 28 433 



