MODERN BIOLOGY 563 



descent partly on the theory of variability and natural selection brought 

 about by the struggle for existence among the variations, and partly on the 

 assumption of the direct influence of environment upon the individual, and 

 the inheritance of the changes thus brought about — that is, a Lamarckian 

 conception. Here at once, in this double explanation, lay the seeds of dis- 

 sension: one could with prejudice emphasize the idea of selection, or with 

 equal prejudice maintain the influence of environment. And this was ex- 

 actly what happened; the period immediately preceding and around the turn 

 of the century witnessed the birth of the two evolutional schools of thought 

 called neo-Darwinism and neo-Lamarckism, whose advocates sought to con- 

 vince the biologists of the absolute validity of their own views. Out of these 

 two main directions there further originated a number of special attempts to 

 explain the causes of evolution, so that the situation in which the doctrine 

 of descent eventually found itself was somewhat chaotic. We shall here de- 

 scribe some features of this internal dissolution of Darwinism. 



In Germany the theory of selection found a highly gifted and power- 

 ful advocate in the person of August Weismann (1834-1914). He studied 

 medicine, being a pupil of Leuckart, who inspired him with an interest for 

 biology. After working for some years as a practitioner he was invited, on 

 account of a useful treatise on the evolution of flies, which he had written 

 in the mean while, to be professor at Freiburg, where he laboured until his 

 death. His special subject was the evolution of the lower animals; in this 

 field he particularly distinguished himself in his studies of the reproduction 

 of the Daphniidas, as a result of which he elucidated the peculiar egg-de- 

 velopment in these crustaceans and the no less curious "cyclic reproduction" 

 that characterizes them. An ophthalmic disease, however, soon precluded 

 him from using the microscope and compelled him to apply himself partly 

 to experimental and partly to purely speculative activities. One result of 

 this was his strange theory of evolution, which placed him among the very 

 foremost of Darwin's successors. 



Weismann's theory of descent and heredity is based, firstly, on his 

 above-mentioned special investigations, and secondly on Nageli's idioplasma 

 theory, referred to above. Nageli had sought for a material substructure for 

 the inherited dispositions, out of which are developed in every individual 

 certain given qualities, and he believed he had discovered it in his hypothe- 

 sis of the idioplasma, which, existing equally in the egg and in the sperm, 

 through their union forms in the new individual the basic material for its 

 special qualities. Weismann, who as a result of his studies and his own re- 

 search work had acquired a deep insight into contemporary cytological 

 knowledge, came for that very reason, when he was forced to devote him- 

 self to purely theoretical speculations, to take up the question of cell-struc- 

 ture as a basis for the evolutional theory that Darwin and his school had 



