'PHYSIOLOGICAL UNITS' 75 



conceptions concerning these processes, (a) from a mere matter of 

 life, and regard the simplest vital units as composed of more or less 

 complex aggregates of chemical molecules, whether they have 

 been spoken of as "physiological units" by Herbert Spencer (1863), 

 as "plastidules" by Haeckel (1875), as "micella" by Nageli (1884), 

 as "pangenes" by de Vries (1889), or as "biogens" by Vervvorn 

 (1894). There are others, howrever, who, while professed evolu- 

 tionists, base their theories upon (b) a particular form of life, the 

 nucleus, and make that, which must be a product of Evolution, the 

 starting point for their theories. These latter views are more 

 especially associated with the name of Weismann (1881-1904) and 

 his gradually developed hypotheses concerning 'ids' and 'deter- 

 minants' as constituents of the nucleus.' 



It will be best for us first to say something concerning these 

 views of Weismann. They are, however, so complicated as to 

 make it difficult to give any adequate account of them in a few 

 words. They can count upon very many firm adherents ; though 

 others are more struck with the number of unproved assumptions 

 and more or less baseless hypotheses which are most skilfully used 

 with the appearance of explaining this or that problem as it 

 arises. Some of Weismann's fundamental positions, however, are 

 so absolutely hypothetical and incapable of verification, if not 

 actually opposed to existing knowledge, as necessarily to instil the 

 gravest doubts as to the value of his theories. They lead him 

 to attribute to 'natural selection' together with his supple- 

 mentary doctrine of ' germinal selection ' (as causes of Evolution) 

 an influence much more profound and far-reaching than was ever 

 claimed by Darwin himself : they lead him to deny the evolu- 

 tional effects of "use and disuse," and, in other words, the 

 "inheritance of acquired characters" — processes which are re- 

 garded by Herbert Spencer, Haeckel and others as real and very 

 important factors in the process of Evolution. 



Weismann postulates the presence in living matter of a special 

 hereditary substance, and this substance he believes to exist in the 

 ' chromatin granules ' which, when aggregated, form the ' chromo- 

 somes ' of the nucleus, as they occur in the germ-cells and 

 sperm-cells, as well as in cells generally. These chromosomes 

 of the nucleus are often band or rod-shaped, more rarely minute 

 and granular, though in the former case the bands or rods are 



' The latest presentation o£ his doctrines is to be found in a recently published 

 work, "The Evolution Theory," 1904. (Transln.) Two Vols. 



