No. 643] GROWTH IN CHROMOSOMES 



173 



single short chromosome fifty units long. It is not to be 

 thought of as a fixed portion of any chromosome. The 

 chromosomes are, or course, not to be considered as 

 marked off into fifty-unit lengths. But the result of re- 

 combination with a large number of factors is approxi- 

 mately the same as if such were the case. Because it 

 brings out the fact which has not always been fully 

 appreciated, that recombination within a chromosome 

 takes place as easily as between different chromosomes, 

 when the distance between the loci is sufficiently great, 

 the term menclel as a measure of chromosome length may 

 have some value. 



In the primitive unicellular organisms it is conceivable 

 that the hereditary substances were not located in a 

 mechanisn as well regulated a^ in the liighcr organisms. 

 As specialization increased, the ,ur()ii]iiiig of factors in 

 chromosomes has undoubtedly lieen of very great evolu- 

 tionary significance. The cliromosonie mechanism has 

 been subjected to natural selection as severely as any 

 external morphological feature and has develo]>ed co- 

 ordinately with sexual i-eproduction — the one to make 

 recoDibinatitni i)()>^sil)l(\ tlio other to make that process 

 orderly. 



Although it is largely speculation it seems necessary 

 to believe that there is some functional relation between 

 the factors associated together in a chromosome or por- 

 tion of a chromosome. There is some evidence for this 

 in the quick and exact return of certain species hybrids 

 to one or tlu' otlici- ]);)ri'iital type. Evidently only those 

 individuals nv-iilriiig tVoiii gametes in which crossing over 

 has not occiint'd altle to live. So far the factors 

 which have bet'ii located -eeni to bo placed at random 



significant relation atnofig them. This in itself may be 

 an indication of an imnu'nso nninhcr of hereditary de- 

 terminers which play a ]Kirt in the organism. For as 

 yet the function of only the relatively vni)erficial factors 

 can be seen. The vitally important ones can not be dis- 

 pensed with and therefore can not be studied except as 

 the lethal factors show some effect in hybrid combination. 



