166 Darwin, and after Darwin. 
the enormous profusion of the extinct species on the 
one hand, and to the immcnse number of known 
fossils on the other, it was incredible that no satis- 
factory instances of specific transmutation should ever 
have been brought to light, if such transmutation had 
ever occurred in the universal manner which the theory 
was bound to suppose. But since Darwin first 
published his great work paleontologists have been 
very active in discovering and exploring fossiliferous 
beds in sundry parts of the world; and the result of 
their labours has been to supply so many of the 
previously missing links that the voice of competent 
criticism in this matter has now been well-nigh silenced. 
Indeed, the material thus furnished to an advocate of 
evolution at the present time is so abundant that his 
principal difficulty is to select his samples. I think, 
however, that the most satisfactory result will be 
gained if I restrict my expcsition to a minute account 
of some few series of connecting links, rather than if 
I were to take a more general survey of a larger 
number. I will, therefore, confine the survey to the 
animal kingdom, and there mention only some of the 
cases which have yielded well-detailed proof of con- 
tinuous differentiation. 
It is obvious that the parts of animals most likely 
to have been preserved in such a continuous series of 
fossils as the present line of evidence requires, would 
have been the hard parts. These are horns, bones, 
teeth, and shells. Thercfore I will consider each of 
these four classes of structures scparately. 
Horns wherever they occur, are found to be of high 
importance for purposes of classification. They are 
