It is evident, then, that in this case of “dimorphism,” the tails of the one group of earwigs (which Bateson calls the “high males”) have either grown _faster_, or have been growing for a longer period of time, than those of the “low males.” If we could be certain that the whole random sample of earwigs were of one and the same age, then we should have to refer the phenomenon of dimorphism to a physiological phenomenon, simple in kind (however remarkable and unexpected); viz. that there were two alternative {106} 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, if we could believe that the two groups of earwigs were _of different ages_, then the phenomenon would be simplicity itself, and there would be no more to be said about it[133]. ―――――――――― 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[134]. {107} _Full-fed Rats._ Age in Length of Length of Total weeks body (mm.) tail (mm.) length % of tail 0 48·7 16·9 65·6 25·8 1 64·5 29·4 93·9 31·3 3 90·4 59·1 149·5 39·5 6 128·0 110·0 238·0 46·2 10 173·0 150·0 323·0 46·4 _Underfed Rats._ 6 98·0 72·3 170·3 42·5 10 99·6 83·9 183·5 45·7 Again as physiologists have long been aware, there is a marked difference in the variation of weight of the different organs, according to whether the animal’s total weight remain constant, or be caused to diminish by actual starvation; and further striking differences appear when the diet is not only scanty, but ill-balanced. But these phenomena of abnormal growth, however interesting from the physiological view, are of little practical importance to the morphologist.