No.2.) SHE HARD PARTS OF THE MAMMALIA. 141 
») 
which possess them? Not only is no explanation offered by the 
Post-Darwinian school, but such progress is, under their hypoth- 
esis, impossible. It is an explanation of odscurus per obscurtus. 
But we are still of the opinion, in spite of Weissman’s theory to 
the contrary, that so long as the germ plasma is subject to 
nutrition, it is subject to influences occurring during the adult 
life of an animal, and it would be an exception to all the other 
tissues were it not so.! 
The ground covered by the present essay is only that which 
paleontological discovery has brought fairly within reach. The 
history of some parts of the skeleton in various types is yet 
unknown, especially as regards their function in extinct forms. 
The solution of these problems must be left for future research. 
It has been found that in all instances where it has been prac- 
ticable to observe living animals, explanations of puzzling struc- 
tures were found in some peculiar habitual movement which they 
exhibit. In this investigation I have been, therefore, greatly 
aided by the Zodlogical Garden of Philadelphia and its able 
superintendent, Mr. Arthur E. Brown. I have also derived 
considerable assistance from the extensive series of photographs 
of animals in motion, by Muybridge, issued under the auspices 
of the University of Pennsylvania; and the analysis of these 
motions in the accompanying text by the distinguished anato- 
mist, Dr. Harrison Allen. 
It must be mentioned at the outset that bone-tissue is plastic, 
especially in the living state, and in time moulds itself upon 
resisting surfaces. The more spongy tissues modify their form 
in accordance with denser, when in contact. As Professor Ryder 
has remarked, even a substance as rigid as the enamel of the 
teeth yields its form to continued pressure and strain, as not 
less rigid rocks and stones are known to do. But in a living 
tissue like bone, the effect of such application of energy is evi- 
dently much greater than in any equally rigid non-living body. 
The metabolism of nutrition is clearly a most important factor 
in the production of the results, rendering the transfer and loca- 
tion of building material possible, which unassisted impacts 
and strains could not accomplish. 
1 4merican Naturalist, March, 1889. 
