234 PROBLEMS OF RELATIVE GROWTH 



the bar-eye series of Drosophila (for details see Goldschmidt, 

 1927 ; and notably Hersch, 1928). 



An interesting case where genes independently control 

 growth-intensity in length and in breadth is afforded by the 

 studies of Sinnott and his associates (see Sinnott and Hammond, 

 1930), who find that fruit-shape in the gourd Cucurbita depends 

 on the interaction of various separate factors of this nature. 

 Sinnott and Durham (1929) find that most of the form- 

 differences depend primarily on the growth of the carpellary 

 tissue, the wall of the ovary and fruit merely following the 

 form-changes induced by the central region. The results of 

 Sinnott (1930). may be interpreted to mean that there exist 

 various genes controlling size and shape independently, but 

 that the shape-factors generally control growth-ratios, and 

 therefore there is a progressive change of shape with absolute 

 size in individual development. 



An instance of a single gene having a differential but appar- 

 ently graded effect on a localized region is found in the house- 

 mouse, where the gene for short ears produces a marked short- 

 ening of the skull, especially in the anterior-palate region ; 

 a very marked diminution in the height just behind the incisors 

 with a slighter diminution posteriorly, and an increase in 

 width, rather more marked posteriorly than anteriorly (Snell, 



1931). 



§ 6. Relative Growth, Embryology, and Recapitulation 



A rather different set of evolutionary problems from those 

 we considered previously has light shed on it by a considera- 

 tion of differential growth-coefficients and the rate-genes which 

 we must postulate to regulate them. These are the problems 

 of recapitulation and of vestigial organs. 



There are numerous cases of recapitulation which cannot, 

 in my opinion, be accounted for along these lines, e.g. the 

 presence of notochord and gill-clefts in the embryos of the 

 highest vertebrates, but there are many others which do receive 

 some explanation from this source. E.g. we find that the 

 relatively long-armed Gibbons have a fetus which, though 

 longer-armed than that of other anthropoids, is relatively 

 shorter-armed than the adult (Schultz, 1926). Presumably 

 the line of least biological resistance in altering proportions of 

 limbs, etc., is to modify relative growth-rates, rather than to 

 modify the original partition of material in the early embryo 

 between organ and rest-of-body. And if this is so, the recapitu- 



