THE TAIL 



In fact I feel sure that I have seen a muskrat swimming slowly by means 

 of the tail alone, while the feet were trailed, but at speed the feet must 

 still be employed. 



In the above mammals the musculature controlling lateral movements 

 of the tail is not as yet appreciably specialized, although it must be in- 

 appreciably so. In other words the muscles must be considerably more 

 fitted for this work than is the case in the nearest terrestrial relatives, 

 but not to the extent that this has as yet had much effect upon gross 

 conformation. All that can now be said is that as this specialization 

 becomes more marked, the muscles controlling lateral motions of the 

 tail will become larger, broader, and there will develop a tendency for 

 muscular simplification and partial fusion. 



I regard it as doubtful if the tail of Potomogale ever presented quite 

 the appearance that we find in most terrestrial insectivores. As al- 

 ready mentioned the case of this genus is somewhat puzzling, but it 

 seems to me probable that its tail was already quite robust at its base 

 when it first took to the water. The tail of this genus is now as perfected 

 a swimming organ as it can probably ever be unless the animal become 

 less dependent upon the land. Swimming is evidently accomplished 

 not by a violent wriggling of the tail, as in the muskrat, but by more 

 circumscribed, although stronger, motions, enabling the animal to slip 

 through the water with remarkable ease and speed. I have never had 

 a specimen for dissection but Dobson (1882) has illustrated its muscu- 

 lature to good advantage. The surprising detail shown is the way in 

 which the gluteal complex has become specialized. One division of the 

 latter has expanded so as to stretch from the knee posteriorly and far 

 over the base of the tail. Clearly this gives the latter added lateral 

 leverage, so that a number of the muscles of the posterior limb can 

 assist oscillations of the tail, either directly, or indirectly as antagonists. 

 The muscles from the pelvis, consisting of Dobson' s ischio- and ileo- 

 caudalis, are both large and long, and indeed there is marked increase 

 in the robustness of all the muscles of the tail proper. Unlike most mam- 

 mals of this size the muscles of the base of the tail are entirely fleshy, 

 rather than chiefly tendinous, and this is the reason for the gradual ex- 

 tension onto the tail of the body contour. 



Before abandoning the subject of mammalian tails that have become 

 flattened transversely it is justifiable to discuss future potentialities and 

 successive steps in their eventual development which seem most likely. 

 This, of course, is pure speculation, but is of great interest, and possi- 

 bilities can at least be mentioned. 



[189] 



