262 OUTLINES OF EVOLUTIONARY BIOLOGY 



the sudden and sporadic reappearance of some ancestral structure 

 which had either been completely lost or very greatly reduced. 

 The elder Pliny has placed it on record that Caesar the Dictator 

 had a horse whose fore feet were like those of a man. This 

 statement evidently refers to a more or less complete return to 

 the ancestral pentadactyl condition. Marsh has given a figure 

 of the fore foot of a horse in which the second digit is fairly 

 well developed, with three perfect phalanges, though not so large 

 as the third, while the first is represented by a splint bone, and a 

 very similar case is exhibited in the Natural History Museum at 

 South Kensington. 



In man the occasional excessive development of the canine 

 teeth is regarded as due to reversion to an ancestral condition 

 similar to that of the anthropoid apes, in which the canine teeth 

 are very large and used as weapons. The occasional occurrence 

 in man of vestigial tail muscles, and even of a short tail, may 

 also be regarded as due to reversion. Whether or not all such 

 cases are capable of being explained on Mendelian principles, as 

 suggested in Chapter XIV, it is impossible to say, but this does 

 not affect their importance as evidence of the truth of the theory 

 of organic evolution. 



Although, in the present chapter, we have so far drawn our 

 illustrations entirely from the animal kingdom, it must not be 

 supposed that the same general principles cannot be equally well 

 demonstrated in the case of plants. Indeed the whole chapter 

 might be re-written entirely from the botanical point of view. 

 Change of function is very well shown, for example, in the 

 " pitchers " of Nepenthes and Sarracenia, formed from modified 

 leaves and serving as traps for catching insects. Convergent evolu- 

 tion is illustrated by the strong superficial resemblance which exists 

 amongst the various Alpine cushion plants belonging to widely 

 different orders, all of which have assumed the form best suited 

 for withstanding the peculiar hardships of their environment. 

 The leaves of the parasitic dodder, reduced to tiny scales (Fig. 

 184), and the staminodes or functionless stamens of many flowers, 

 again, might well serve as examples of vestigial structures. 

 Considerations of space, however, forbid us to pursue this fertile 

 line of inquiry any further. 



