HETEROGONY AND PHYSIOLOGY 227 



parasites upon smaller birds, like Cuculus canorus, and in 

 the high values of b found for most forms with precocial 

 young. 



In regard to the physiological side of the problem, it is im- 

 portant to distinguish two very different ways in which change 

 in relative size of a part may be brought about. We have 

 already discussed certain implications of this distinction, but 

 have not set it forth in precise form. (See also pp. 257-8.) 



(1) An organ may be capable of functional adjustment in 

 size. The best example of this is the heart. 



(2) An organ may possess a differential coefficient of growth- 

 partition, which operates wholly or largely irrespective of 

 functional demands, e.g. the antlers of deer, the large chela 

 of male Uca, the limb of Ambly stoma tigrinum grafted on to 

 A. panctatum, the pituitary of rabbits (Robb, 1929), etc. 

 Increase of absolute size of an animal will in both cases bring 

 about an alteration of relative size of the part, but in the 

 first case owing to functional hypertrophy, in the second owing 

 to the specific growth-intensity of the organ, which in its turn 

 is presumably due to a specific growth-promoting substance. 

 And doubtless in many cases, both mechanisms will operate 

 simultaneously. 



Some cases of so-called functional hypertrophy may turn 

 out to belong to the second group, e.g. the enlargement of the 

 remaining testis after unilateral castration is probably not 

 functional in the usual sense at all, but depends upon the exist- 

 ence of a specific partition-coefficient for testis-tissue relative 

 to total size. (See Domm and Juhn, 1927, and also p. 257.) 



The distinction, as I have suggested, is usually not an 

 absolute one. If one prevents a limb from exerting its normal 

 function, e.g. by tying it up at birth, its growth will be markedly 

 subnormal. With regard to heart-weight, etc., it is a priori 

 almost certain that the heart, in addition to its capacities 

 for marked functional size and regulation, possesses a primary 

 partition-coefficient which will differ from species to species. 

 What we can say, however, is that in some cases the differences 

 observed are due solely or mainly to functional regulation, 

 in others solely or mainly to differences in inherent growth- 

 capacity. 



But there are cases in which organs attain their definitive 

 relative size entirely owing to one or other of the two methods. 

 The size of the organs of holometabolous insects must depend 

 entirely on their growth-partition coefficient ; while it is 



