CH. xvi] STRUCTURE AND FUNCTION 257 



without this aspect of the subject to distract them, cytolo- 

 gists are continually driven to ask for explanations in terms 

 of functional or physical causes. 



Very frequently the attempt to correlate cytological 

 phenomena with the functions of the organism as a whole 

 has been made in the opposite direction to that indicated 

 above; instead of the cytologist looking for elucidation of 

 his discoveries in terms of function, the investigator of 

 function has turned to cytology for explanation of the facts 

 that he observes. But in most of the chief advances that 

 have been made, the first steps have been taken on the 

 purely cytological side. This is seen especially in the sub- 

 jects that have occupied cytologists more than any others 

 in the last few years the material basis of hereditary trans- 

 mission and of the determination of sex. The foundations 

 of modern ideas on the localisation of the factors for inheri- 

 ted characters was laid by WEISMANN, and though much 

 of the theoretical superstructure built by him on insufficient 

 factual evidence has proved unsound, his basal idea of the 

 relation between these factors and the chromosomes is 

 continually receiving fuller recognition. And WEISMANN 

 was led to his idea of the germ-plasm directly from his work 

 on the early origin of the germ-cells, and to his hypothesis 

 of the function of the chromosomes from researches of 

 cytologists on their behaviour in the maturation divisions. 

 Similarly, the idea that the separation of the chromosomes 

 in maturation is responsible for the segregation of Mendelian 

 characters in the germ-cells of a heterozygote arose from the 

 investigations of WILSON, SUTTON and others on the sperm- 

 atogenesis of Orthopterous and Hemipterous Insects, and 

 the latest development of this theory the chiasma type 

 hypothesis of MORGAN is based on the purely cytological 

 investigations of JANSSENS. The same thing is true of the 

 D. c. 17 



