CHROMOSOMES AND HEREDITY — DARLINGTON 423 



EVOLUTION AND THE CELL 



It will be seen here that an evolutionary point of view was begin- 

 ning to force itself into my argument. 



There were several reasons why this should have happened to the 

 student of chromosomes, by no stretch of the imagination but by hard 

 necessity. While the experimental breeder could sort out linkage in 

 one species, the chromosomes could reveal chiasmata in a hundred 

 species and in every group of plants and animals. While the experi- 

 mental breeder himself decided how his plants or animals should 

 breed, the chromosome man had to pick up his cells and discover how 

 nature had bred them, and why, and with what effect. These were 

 two reasons. But a third was even larger. It was that through 

 the chromosomes there is continuity between successive generations. 

 To the naturalist and to the experimental breeder the organism is an 

 independent discontinuous entity. To the cytologist it is part of a 

 continuous process. Cell division is always a step between the past 

 and future : it is always adapted to meet conditions which do not yet 

 exist, to produce progeny which are irrelevant to their parent's success. 



Oenothera first brought this home to me. In its evolution there had 

 been interchanges between different chromosomes, each of which 

 succeeded by virtue of its selective advantage over its predecessors. 

 But success depended on whether plants were inbred or outbred. Thus 

 the hereditary mechanism and the sexual mechanism, the means of 

 distributing and recombining differences and the means of bringing 

 them together, must be bound up together in one system, a genetic 

 system. In a genetic system crossing-over of chromosomes is no good 

 without crossing of germ cells, without outbreeding. The two proc- 

 esses must be adjusted to one another. They must also be adjusted 

 to the needs, not of the individual, but of the breeding group and, 

 more particularly, of its posterity. 



A second example was in the male Drosophila^ with its suppression 

 of crossing-over. In these flies the male, I found, had contrived an 

 anomalous kind of meiosis without crossing-over [10]. The chromo- 

 somes paired and separated without needing to form chiasmata. The 

 breeding and the chromosome observations thus agreed. But how had 

 an otherwise universal rule come to break down — and break down in 

 the very species of organism in which the rule was first brought 

 to light? 



The reason is obvious as soon as it is pointed out. In the verte- 

 brates or flowering plants the genes in the chromosomes are recombined 

 once in every sexual generation. This may be once in 10 months or 

 10 years. But in the short-lived flies it happens once in 10 days. 

 That is why from the whole animal kingdom Alorgan chose to work 



