EVIDENCES OF EVOLUTION. -85 
of Species (1859), made it current intellectual coin. By’ 
his work, and by that of Spencer, Wallace, Haeckel, Huxley, 
and many others, the doctrine of descent, the general fact 
of evolution, has been established, and is now all but 
universally recognised. ; 
The chief arguments which. Darwin and others have 
elaborated in support of the doctrine of descent, according 
to which organisms have been naturally evolved from simpler 
forms of life, may be ranked under three heads—(a) struc- 
tural, (4) physiological, (¢) historical. 
Evidences of evolution.—(a) Structural.—Some say that 
there are over a million living animals of different species. 
In any case, there are many myriads. These species are 
linked together by varieties which make strict severance 
often impossible ; they can be rationally arranged in general 
orders, families, and classes, between which there are not a 
few remarkable connecting links ; there is a gradual increase 
of complexity from the Protozoa upwards along various 
lines of organisation; it is possible to rank them all ona 
hypothetical genealogical tree (Fig. 18). A little practica, 
experience makes one feel that the facts of classification 
favour the idea of common descent. 
Throughout vast series of animals we find in different 
guise essentially the same parts twisted into most diverse 
forms for different uses, but yet referable to the same funda- 
mental type. It is difficult to understand this “adherence 
to type,” this “homology” of organs, except on the theory 
of natural relationship. 
There are many rudiméntary organs in animals, especially 
in the higher animals, which remain very slightly developed, 
and which often disappear without having served any 
apparent purpose. Such are the “gill-slits” or “ visceral- 
clefts” in Reptiles, Birds, and Mammals, the teeth of young 
whalebone whales, the pineal body (a rudimentary eye) in 
Vertebrates. Only on the theory that they are vestiges of 
structures which were of use in ancestors are these rudiments 
intelligible. They are relics of past history, comparable, as 
Darwin said, to the unpronounced letters in many words. 
(2) Physiological —Observation shows that animals are to 
some extent plastic. In natural conditions they usually 
exhibit some measure of changefulness from generation to 
