Morphological Variation and Its Causes in A. tignnnm 39 



The modification of the form of the head in adults by certain 

 artificial conditions is interesting. For example, I should class 

 permanent aquatic life for this species as artificial, because of, 

 among other reasons, the modification in the species which it pro- 

 duces, especially if the animals are taken when young and highly 

 fed. Certain characters of the head dating from larval life are 

 not obliterated by even this treatment; long heads remain long 

 and short heads short. But other characters are developed which 

 modify the appearance of the animal very greatly and which, 

 coupled with equally conspicuous color changes,^ render the ani- 

 mals well-nigh unrecognizable as A. tigrinum. Zoologists to 

 whom I have displayed such animals have never been willing to 

 hazard a guess as to their species. Notwithstanding variations 

 in degree and some variety in result, these changes are always 

 in the same general direction : the head is broadened and flat- 

 tened, and something of the same truncation usually appears that 

 was shown to occur in the larva under similar conditions. As to 

 the broadening of the head, a fine typical example is shown in 

 figure 3, plate IV, an animal that had been kept continually in 

 shallow water for over a year. When the experiment began the 

 specimen was an ordinary small-sized young adult, having little 

 resemblance to the figure. It will be noticed that this head is 

 even broader in proportion to length than that of the unusually 

 large-headed adult beside it, which owed its form directly to the 

 conditions of its larval existence. This broadening of the head 

 by life in water does not necessarily imply a robust animal ; not 

 a few instances occur of the opposite combination. Figure 6, 

 plate V, shows a dorsal view of the head of the animal in figure 

 2 of the same plate, although this view of the head was taken be- 

 fore the animal was sexually mature or had reached the length 

 shown in figure 2. The animal was, however, extremely slender 

 in both body and tail, while- the head as shown became almost 

 round in outline. The most extreme instance of this lateral de- 



^The writer has in preparation a monograph on the color variation of 

 A. tigrinum, which will explain, not only such peculiarities as are shown 

 in the plates of this article, but many others, including several apparently 

 new types. It will also fully explain the differences in coloration on the 

 different portions of the animal's bodv. 



235 



