THE RECAPITULATION THEORY 181 



are faced with hard enamel, so that the action of the upper teeth upon 

 the lower keeps the cutting edges extremely sharp ; these teeth do not 

 form roots, but continue to grow throughout the lifetime of the animal. 

 Between the chisel-like incisors and the grinding teeth, there is a long 

 toothless gap, which, we assume, was, in the ancestors of the rodents, 

 occupied by the second and third incisors, the canine and two or more 

 grinders. This conclusion is justified by the facts of embryology; 

 for instance, in the embryo of the squirrel several of the missing teeth 

 are begun as distinct tooth-germs, but fail to develop, never cut the 

 gum and are resorbed before birth. 



All available evidence points to the conclusion that birds are 

 descended from reptiles, a conclusion which is especially strengthened 

 by the facts of palaeontology and will be examined more at length 

 in the following lecture. Such a descent explains many otherwise 

 puzzling features in the ontogeny of birds, in which reptilian charac- 

 teristics appear in transitory fashion and are either modified so as to 

 take on typically bird-like character, or are suppressed altogether. A 

 remarkable example of this is the formation of rudimentary teeth in 

 certain embryonic birds, followed by their resorption and disappear- 

 ance before hatching. 



It can hardly be contended that these rudimentary structures, 

 which are confined to the embryonic stages of development and of 

 which no trace remains in the adult, are so indispensable to the 

 processes of ontogeny, that they were specially created to serve this 

 temporary purpose. For such a contention there is not a particle 

 of evidence and the theory of evolution, which regards these structures 

 as useless remnants, due to inheritance from ancestors in which the 

 structures are functional, offers much the most satisfactory solution 

 of the problem that has yet been suggested. 



Embryology further shows that evolution is not invariably an 

 advance from lower and simpler to higher and more complex types, 

 but may be by way of degeneration and degradation. The adoption 

 of a parasitic mode of life is very apt to cause such degradation, and 

 some very remarkable instances of the degeneration of parasites have 

 been observed. An instructive example that may be cited is that of 

 Sacculina, a nondescript creature that is parasitic on certain species 

 of crabs. The parasite is attached to the body of its victim, under- 

 neath the tail, by means of root-like fibres which penetrate and ramify 

 throughout the interior of the crab. The root-like fibres absorb nutri- 

 ment and convey it to the body of the parasite, which is reduced to a 



