Morphological Variation and Its Causes in A. tigrinum 31 



supply. The contrast was surprising. These larvae were feeding 

 by sight. A single daphnid was seized at a time, and this only 

 when it -chanced to come into exactly the appropriate position : 

 the mouth was opened so slightly and the prey seized so deftly 

 that the whole movement was inconspicuous and trifling. Here 

 there was suggested a possible explanation of the different forms 

 of head. The straining of the gape to the utmost, the expansion 

 of the whole buccal apparatus, and the continued repetition of 

 the process might well be the cause of the head expansion in the 

 pond larvae, while the gingerly nabbing of single daphnids by the 

 scarcely opened jaws of the specimens kept in the light might 

 give the narrow head and insignificant mouths of the others. Ad- 

 ditional confirmation was necessary, however, and the following- 

 spring I introduced young larvae into aquaria kept in the dark. 

 I supplied these again with daphnids, although in but moderate 

 quantity, in order to stimulate the animals to as continuous feed- 

 ing and as much mouth expansion as possible. The result was 

 the rapid development of larger mouths and broader heads than 

 I had ever produced by ordinary feeding, even when this had 

 been excessive and had resulted in very robust types. In general, 

 too, I had always been surprised that my laboratory larvae, fed on 

 abundant and highly nutritious food, should develop . narrower 

 heads than nearly all of those from surrounding ponds. The ex- 

 planation now became clear. And, moreover, the investigation 

 of a number of ponds in August showed a surprising correlation 

 of broader and broader heads with muddier and muddier water. 

 The broadest headed daphnid-feeders I obtained were developed 

 in a prairie cattle-wallow where the water was kept continuously 

 agitated until it was black with mud. All of the other ponds ad- 

 mitted some light during at least the longer intervals between 

 rains. 



This principle, that the head is broadened by random feeding 

 in the dark and narrowed by easy feeding under conditions that 

 allow of the use of the sense of sight, is of course complicated 

 more or less by the results of slow or rapid growth and of free- 

 swimming or bottom-crawling habits, for these latter affect the 



227 



