106 THE RATE OF GROWTH [ch. 



values, very different from one another, for the mean velocity of 

 growth, and that the individual earwigs varied around one or 

 other of these mean values, in each case according to the law of 

 probabilities. But on the other hand, it we could beheve that 

 the two groups of earwigs were of different ages, then the pheno- 

 menon would be simphcity itself, and there would be no more to 

 be said about it*. 



Before we pass from the subject of the relative rate of growth 

 of different parts or organs, we may take brief note of the fact 

 that various experiments have been made to determine whether 

 the normal ratios are maintained under altered circumstances of 

 nutrition, and especially in the case of partial starvation. For 

 instance, it has been found possible to keep young rats alive for 

 many weeks on a diet such as is just sufficient to maintain life 

 without permitting any increase of weight. The rat of three 

 weeks old weighs about 25 gms., and under a normal diet should 

 weigh at ten weeks old about 150 gms., in the male, or 115 gms. 

 in the female ; but the underfed rat is still kept at ten weeks old 

 to the weight of 25 gms. Under normal diet the proportions of 

 the body change very considerably between the ages of three and 

 ten weeks. For instance the tail gets relatively longer ; and even 

 when the total growth of the rat is prevented by underfeeding, 

 the form continues to alter so that this increasing length of the 

 tail is still manifest |. 



* I do not say that the assumption that these two groups of earwigs were of 

 different ages is altogether an easy one; for of course, even in an insect whose 

 metamorphosis is so simple as the earwig's, consisting only in the acquisition of 

 wings or wing-cases, we usually take it for granted that growth proceed? no more 

 after the final stage, or "adult form" is attained, and further that this adult form 

 is attained at an approximate^ constant age, and constant magnitude. But even 

 if we are not permitted to think that the earwig may have grown, or moulted, 

 after once the elytra were produced, it seems to me far from impossible, and far 

 from unlikely, that prior to the appearance of the elytra one more stage of growth, 

 or one more moult took place in some cases than in others: for the number of 

 moults is known to be variable in many species of Orthoptera. Unfortunately 

 Bateson tells us nothing about the sizes or total lengths of his earwigs; but his 

 figures suggest that it was bigger earwigs that had the longer tails ; and that the 

 rate of growth of the tails had had a certain definite ratio to that of the bodies, 

 but not necessarily a simple ratio of equality. 



■j- Jackson, C. M., J. of Exp. Zool. xix, 1915, p. 99; cf. also Hans Aron, Unters. 



