INCONSTANCY OF FORM 39 



limits of normal variation, of which more later) solely upon 

 its absolute size, not upon the length of time it has taken 

 to reach that size, nor upon changes in any other external 

 variable. The modifications of and exceptions to this statement 

 we shall consider later ; here we can accept it as our first 

 general rule. 



As immediate corollary of this we have the fact that only 

 animals in which all organs are growing at the same rates 

 will preserve their form unchanged with increase of size ; and 

 this is as much as to say that no animal will keep its form 

 identical while increasing in size, for it appears highly improb- 

 able that any animal will be found in which some organs do 

 not grow at a different rate from the body as a whole. And 

 even if for the moment we stick to external form, and further 

 if we only consider quite large differences in growth-activity, 

 we shall find many animals in which, as in the male fiddler- 

 crab, the only constancy of form is the constancy of its mode 

 of change. This is less obvious and in some ways less important 

 in the higher animals (notably almost all mammals and birds 

 among vertebrates and almost all insects and spiders among 

 arthropods), in which growth ceases at a definite size, and 

 there supervenes an adult stage of constant size and often 

 of long duration. For here we can often afford to consider 

 only the adult forms, in which the proportions of form have 

 been fixed by the cessation of growth. It is the limitation 

 of form at a fixed absolute size which confers this convenience 

 upon the systematist and the morphologist. 



Even here, however, as we shall see in detail later, the rule 

 has many applications. To take the most obvious case, the 

 absolute size at which growth ceases may be altered by treat- 

 ment such as feeding ; in such case, the permanently stunted 

 individual will approximate in proportions to a normal juvenile 

 stage, the well-fed, abnormally large specimen will have pro- 

 portions not met with at all among the normal population of 

 adults. 1 It is, in other words, a mere biological accident that 

 adult proportions, even in species with limited growth, are 

 relatively fixed ; and to neglect the fundamental fact of change 



1 As a matter of fact, the degree of development (including growth) 

 of a higher vertebrate appears to be simultaneously dependent upon 

 at least two variables, size and age. This is well shown by Appleton 

 (1925, see his chart 3) as regards the degree of ossification in new- 

 born rabbits ; and by Jackson (1925) for young rodents stunted by 

 underfeeding. Jackson's results are discussed further in Chapter VI. 

 See also the work of Adolph (p. 258). 



