OTHER THEORIES OF EVOLUTION 337 



is a suggestion that at the offset growth may be exag- 

 gerated to subserve adaptive ends. Examination of three 

 special cases, however, shows us that the adaptive circum- 

 stances are neither established nor even plausibly suggested. 

 Matthew's theory of the origin of the Machaerodont canine 

 breaks down on two cardinal points. We arc, on the other 

 hand, impressed by the analogy between individual and 

 phyletic hypertely — between (e.g.) the production of excessive 

 osseous material as the result of internal physiological dis- 

 turbance in the individual and similar excessive growth 

 phenomena in phyletic series. 



(2) Huxley (1932, full bibliography) has recently put 

 forward an explanation of orthogenetic phenomena which 

 depends on particular studies of ' heterogonic ' growth. These 

 studies, and in particular Huxley's empirical formula for ex- 

 pressing ' constant differential growth-rate,' need not be dis- 

 cussed very fully for our present purpose. When an animal 

 increases in size its parts do not all increase at the same rate, 

 and in particular the size of some structures increases at a very 

 much more rapid rate than the rest of the organism. Usually 

 there is with increasing size an increase in the relative size of 

 a part, so that the parts of a large animal are relatively larger 

 than those of a small one. Huxley has investigated these rates 

 and found them susceptible to formularisation. He has also 

 shown that such differential growth-rates tend to be associated 

 with growth gradients culminating in a growth centre. The 

 whole architecture of the body is permeated with such gradients, 

 each producing special effects and combining with each other. 

 The net result of growth-rates combined with growth gradients 

 is not, of course, always the same, and animals of the same 

 size do not necessarily have their various parts of the same size. 

 The Roe-deer's antlers, e.g., unlike those of the Red Deer, 

 show a negative heterogony — i.e. a decrease of relative antler 

 weight with increase of absolute body weight among adult 

 males (Huxley, I.e. p. 46). Now, as we have said, Huxley's 

 formularisation of these facts is purely empirical. We know 

 very little about the origin of differential growth-rates. 

 Naturally, when we learn that one chela of the Fiddler Grab 

 (Uca) shows marked heterogony in the male and not in the 

 female, we assume that there is some functional explanation 

 of the difference (p. 332). Huxley suggests that the negative 



