Morphological Variation and Its Causes in A. tigrinum 27 



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 in different ways, it became eyiclent that certain forms of head 

 were correlated, with partial if not entire regularit}-, with certain 

 conditions of development. And finally, after three seasons' work 

 and observation, the causes of all the more familiar characters 

 were, I think, worked out with fairly demonstrative completeness. 



Omitting for the present the most singular and extreme modi- 

 fications of the head, due, as we shall see later, to cannibalism, I 

 will speak of some of the chief variations and of the larval habits 

 and environing circumstances which produce them. 



To begin with, a small and slender head is usually produced by 

 the same influences which produce a slender body and tail, al- 

 though, as we shall see, this need by no means be whollv the case. 

 But under normal conditions it is so. Light nutrition and a free- 

 swimming habit tend to produce at least narrow muzzles and 

 usually narrow heads. Figure 4, on plate II, shows this to a mod- 

 erate degree (some heads and muzzles are, however, mvich nar- 

 rower than this) when contrasted with the other two larvae on 

 the same plate, figure 2 representing a very rapidly grown speci- 

 men (photographed in August; eggs deposited probably in April), 

 while figure i shows a specimen in which moderate growth had 

 been followed by excessively heavy feeding on daphnids and 

 chironomous larvae. The form of the head depends chiefly upon 

 the configuration of the skull itself, but secondarily upon that of 

 the so called parotid glands. These latter are subject to the great- 

 est variations, not only among adults but, as plate II shows, 

 among larvae. And I find that, from the larvae they are trans- 

 mitted directly to the ^dult, in which they are, on the whole, per- 

 manent under ordinary conditions. The partial exceptions to this 

 permanence are : first, that metamorphosis reduces them to some 

 extent, as may be seen by comparing figure 2, plate II, with the 

 same animal after metamorphosis, shown in figure 2, plate IV ; 

 second, severe starvation reduces the parotids to a considerable 

 extent, especially in the young, and conversely, excessive feeding 

 may restore them or even now and then develop them beyond 

 their original size. Nevertheless, these structures tend to be as 

 permanent in the adult as are most other external characters, and 

 with them as with so many other organs it is during the larval life 



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