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 embry onic 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 degene ration 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 thecrab. The root-like fibres absorb nutri- 
ment and convey it to the body of the parasite, which is reduced to a 
