DIMORPHISM AND POLYMORPHISM 363 



brown and blue scales of the LycatiidcE ; and because in inany^ 

 and perJiaps in niost cases, the dimorphic parts correspond only 

 partially or not at all. 



The degree of sexual diiferentiation is very different in the 

 various groups of the animal kingdom. In the lower and 

 higher Crustacea the males commonly possess more ' olfactor\' 

 setie ' on their antennae than do the females. In the large 

 water-flea, Leptodora hyalina, for instance, the anterior antennae 

 are represented in the female by short stumps provided with 

 five olfactory setae, while the male has long rod-like feelers, 

 which bear about eighty setae. This difference evidently cannot 

 be referred to one double determinant. Each olfactory seta 

 must be represented by a special determinant in the germ- 

 plasm ; even if the first five corresponded, and the differences 

 between them could be attributed to the double determinants, 

 more than seventy determinants for these setae — which occur in 

 the male sex only — would still be left, quite apart from those for 

 the feeler itself. These seventy determinants are not double, 

 because the corresponding parts are absent in the female, and 

 consequently two groups of determinants must exist side by side 

 in the germ-plasm, — those, namely, for the antennae in the male 

 and in the female, — only one of which becomes active in each 

 case. We might imagine that the two groups pass, in close 

 proximity to one another, through the cell-series of embryogeny 

 up to the formation of the rudiments of the antennae, and then 

 become separated, the inactive group remaining in an • undif- 

 ferentiated ' cell at the base of the feeler, while the other causes 

 the development of the antennae of the sex in question by 

 continued cell-division. 



The same must be true in numerous other and much more 

 complicated cases. If a tail feather in a male humming- 

 bird, for instance, is six times as long as the corresponding 

 feather in the female, the colour of the former being of a 

 brilliant ultramarine, and that of the latter greyish, we must 

 assume that two groups of determinants are present, which 

 differ in number and in nature. The two groups are situated 

 close together in the germ-plasm, pass through the same cell- 

 series in ontogeny, and ultimately reach the same part of the 

 skin covering the last caudal vertebra. Here, however, one of 

 them remains inactive, the other alone causing further cell-divi- 

 sions and the consequent development of a feather to take place. 



