268 THE RATE OF GROWTH [ch. 



menon of a " gynandrous " feather, male and female on two sides 

 of the same vane*. 



While unequal or differential growth is of pecuhar interest to 

 the morphologist, rate of growth pure and simple, with all the 

 agencies which control or accelerate it, remains of deeper importance 

 to the practical man. The live-stock breeder keeps many desirable 

 quahties in view: constitution, fertility, yield and quahty of milk 

 or wool are some of these; but rate of growth, with its corollaries 

 of early maturity and large ultimate size, is generally more important 

 than them all. The inheritance of size is somewhat complicated, 

 and limited from the breeder's point of view by the mother's 

 inability to nourish and bring forth a crossbred offspring of a breed 

 larger than her own. A cart mare, covered by a Shetland sire, 

 produces a good-sized foal; but the Shetland mare, crossed with 

 a carthorse, has a foal a little bigger, but not much bigger, than 

 herself (Fig. 83). In size and rate of growth, as in other qualities, 

 our farm animals differ vastly from their wild progenitors, or from 

 the " un-improved " stock in days before Bake well and the other 

 great breeders began. The improvement has been brought about 

 by "selection"; but what lies behind? Endocrine secretions, 

 especially pituitary, are doubtless at work; and already the stock- 

 raiser and the biochemist may be found hand in hand. 



If we once admit, as we are now bound to do, the existence of 

 factors which by their physiological activity, and apart from any 

 direct action of the nervous system, tend towards the acceleration 

 of growth and consequent modification of form, we are led into wide 

 fields of speculation by an easy and a legitimate pathway. Professor 

 Gley carries such speculations a long, long way: for He saysf that 

 by these chemical influences "Toute une partie de la construction 

 des etres parait s'expliquer d'une fayon toute mecanique. La forteresse, 

 si longtemps inaccessible, du vitahsme est entamee. Car la notion 

 morphogenique etait, suivant le mot de Dastre J , comme ' le dernier 

 reduit de la force vitale'." 



* See an interesting paper by Frank R, Lillie and Mary Juhn, on The physiology 

 of development of feathers: I, Growth-rate and pattern in the individual feather. 

 Physiological Zoology, v, pp. 124-184, 1932, and many papers quoted therein. 



f Le Neo-vitalisme, Revue Scientifique, March 1911. 



X La Vie et la Mart, 1902, p. 43. 



