920 



SCIENCE 



[N. S. Vol. XLI. No. 1069- 



Without attempting to analyze these dif- 

 ferent books, which would require more 

 time and space than is here available, we 

 may proceed at once to a summary of his 

 more important contributions to the theories 

 of evolution and heredity. 



All his theories, of both heredity and 

 evolution, center in what he called the 

 "germ-plasm," that particular part of the 

 germ-cells which serves to carry over from 

 generation to generation the inheritance 

 factors. This germ-plasm was held by 

 "Weismann to be absolutely continuous from 

 the present generation back to the earliest 

 generations of living things; it was abso- 

 lutely distinct from the somatoplasm of the 

 body and the latter could never become 

 germ-plasm ; it was almost perfectly stable, 

 undergoing practically no changes except 

 such as came from the mixing of different 

 kinds of germ-plasm (amphimixis) in sex- 

 ual reproduction. 



These views as to the nature of the germ- 

 plasm underwent some modification as the 

 result of criticism. Weismann was forced 

 to admit that the distinctness and stability 

 of the germ plasm were not absolute, but 

 in spite of all criticism he was able to main- 

 tain that the germ-plasm was relatively very 

 distinct from other plasms and very stable 

 in organization, and this is now admitted by 

 aU persons acquainted with the subject. 



His views as to the separateness of soma- 

 toplasm and germ-plasm, of body cells and 

 germ cells, and the mortality of the former 

 and potential immortality of the latter, led 

 him to regard organisms in which this dis- 

 tinction does not exist (many protozoa and 

 protophyta) as potentially immortal. With 

 a keenness of insight which was not appre- 

 ciated at the time, but which has been eon- 

 firmed by recent work, he reasoned that 

 "conjugation like food and oxygen may be 

 conditions of life, but immortality does not 

 rest on the magic of conjugation any more 



than on food or ox;ygen. ' ' Again he antici- 

 pated the most recent opinions when he 

 held that death is not a necessary correla- 

 tive of life, but rather the result of higher- 

 differentiation. In short, as Minot said, 

 "Death is the price we pay for our differ- 

 entiation. ' ' On the other hand, his attempt 

 to explain the origin of death as a direct 

 adaptation due to selection was probably 

 a mistaken one. 



As to the location of the germ-plasm in 

 the sex cells Weismann maintained that it 

 was to be found in the chromatic substance 

 of the nucleus. He held that the chromo- 

 somes ("idants") were composed of^ 

 smaller units, the chromomeres ("ids"), 

 and that the latter were composed of 

 "determinants" or inheritance units, while 

 the most elementary units of life he called 

 "biophores." Both chromosomes and 

 chromomeres are visible structures of the 

 ceU. Determinants and biophores are ultra- 

 microscopic in size, but recent work on 

 heredity and development has shown that 

 there is good evidence of the existence of 

 such units. All recent work in genetics is 

 based upon the hypothesis that there are 

 units or factors or determiners in germ 

 cells which condition the development of 

 adult characters, and though there may be- 

 minor differences between these determiners 

 of modern genetics and the determinants of 

 Weismann, no one can fail to note the 

 genetic connection and the family resem- 

 blance between the two. 



His prediction on purely a priori grounds 

 that one of the maturation divisions in the 

 formation of the egg and sperm should be a 

 "reduction division" whereby the chromo- 

 somes of the sex cells should be reduced to 

 half the number present in the somatic 

 cells, whereas all other cell divisions should 

 be "equation divisions" in which the 

 chromosomes should divide equally, was al- 

 most as brilliant an example of scientific- 



