e 3 2 PROBLEMS OF GENETICS 



ferences in size and proportions which he describes can only be 

 appreciated by reference to his figures. They affect almost all 

 features of bodily organisation. These striking differences he 

 looks upon as brought about by differences in nutrition, " diversi- 

 ties in habitual locomotion," and diversity in the age at which 

 metamorphosis occurs, and to sexual difference. Apart from 

 sexual difference he regards the chief distinctions, in brief, as 

 " acquired variations of the larva." 



As an example he gives the great elongation of some of the 

 forms as "due first to slow growth, second to the free-swimming 

 habit, third to the prolongation of larval life, and finally to the 

 assumption of sexual maturity as males," either in the branchiate 

 or non-branchiate condition. He describes the rapid growth of 

 some and the slow growth of others. A larva of intermediate 

 type may grow about a centimeter a month, but a rapidly growing 

 specimen may grow more than four times as much. The slower 

 rate of growth may, he says, be induced by winter feeding, and 

 other treatment. 20 



When, however, he goes on to describe the influences which 

 he regards as exerted by the habit of freely swimming, I am led 

 to wonder whether after all in most of these illustrations, the 

 primary distinctions are not in reality genetic. " Specimens 

 raised in the same aquarium or in similar aquaria, side by side 

 with all conditions as uniform as it is possible to make them, 

 seldom fail to furnish striking examples of broad -headed, short- 

 bodied, and short-tailed types which are habitually found at the 

 bottom, while others, slender and elongated, are free swimmers, 

 and maintain themselves in almost as continual suspension and 

 motion as does a gold fish." Later, again, he writes, " Yet despite 

 the uniformity of these favourable conditions, the larvae soon 

 began to split up into two noticeably distinct groups, the one of 



20 In connexion with this case I would refer the reader to some remarkable 

 observations of Dr. T. A. Chapman on various types of larvae which he reared 

 from the moth Arctia caja (Ent. Rec, IV, 1893, p. 265, and following parts). From 

 a single mother he raised a great diversity of forms, some which fed up rapidly 

 and passed through their development without assuming certain stages, and others 

 which were, as he called them, "laggards," moulting more times than their brethren 

 and developing at a much slower rate. It is greatly to be hoped that such a case 

 may be critically investigated by analytical breeding. 



