INCONSTANCY OF FORM 41 



the animals under standard conditions, since the work of 

 Sumner and of Przibram has shown that increased temperature 

 causes an increased relative size of appendages in rodents. 



It is true the change will not usually be of the same extent 

 after sexual maturity, for although the changes in absolute 

 size may be greater between maturity and death than in 

 the period from the post-embryonic or post-larval phase to 

 maturity, yet the fraction of total growth which takes place 

 after maturity is always a good deal less, if measured by the 

 true criterion, namely the amount of multiplication of initial 

 size. For the fiddler-crab, for instance, the pre-maturity 

 multiplicative increase in weight is about 250-fold, the post- 

 maturity increase about three- to four-fold, though the absolute 

 (additive) increases are roughly as 1 to 2-5. None the less, 

 the post-mature alterations may be very considerable. In the 

 male fiddler-crab, after his attainment of sexual maturity, the 

 proportion of the weight of the large chela increases from 

 43 per cent, of rest-of-body weight to nearly 62 per cent. — 

 an increase of nearly 45 per cent, in relative size. 



No two male Uca pugnax have the same proportions unless 

 they happen to be of the same absolute size : any diagnosis 

 made on the basis of percentage measurements of chelae (and 

 also, though much less markedly so, for other organs such 

 as the pereiopods) would be valueless. But in spite of the 

 fact that the form of the animal is continually changing, it 

 does so in an orderly way ; and though percentage values 

 for the limbs have no diagnostic significance, the constants 

 in the growth-ratio formula are true specific characters. In 

 a word, the systematist needs algebra as well as arithmetic 

 in making any diagnoses based upon the size of parts of the 

 body. 



Note. — S. A. Allen (1894), Amer. Mus. Nat. Hist. Bull., 6, 233, also 

 finds a progressive change of proportions in a rodent (see p. 40). 

 Neotoma shows a steady increase of dolichopy and dohchocephaly 

 with increase of absolute size. 



