Volume III, September, 1889. Number 2. 
JOURNAL 
OF 
MO R Finn CGY": 
THe MECHANICAL "CAUSES, OFP THE DEVELOP- 
MENT OF THEE Akh BAR TS° OF 
THE MAMMALIA. 
BE. DNCOPRE: 
ALTHOUGH three-quarters of a century have elapsed since 
Lamarck formulated the causes which produce variation in 
organic beings, but little has been done towards tracing the 
immediate action of those causes. The father of organic evolu- 
tion ascribed some of these modifications of structure to changes 
in the environment, some to the motions of the organic being, 
and others to both combined.! Spencer in 1865? devoted a short 
chapter to the effect of motion in producing variations, and speci- 
fied the mechanical effect of flexure in producing segmentation 
of the vertebral column. The present writer, in 1871,° insisted 
on the importance of motion as a factor in determining growth, 
and in 18724 approached the subject more definitely in the fol- 
lowing language: ‘The first physical law is that growth force 
... must develop extent in the direction of least resistance, and 
density on the side of greatest resistance.” In 1877 Ryder fur- 
ther applied the principle of motion to the origin of structural 
changes in the feet of Mammalia, in the following language :° 
1 Philosophie Zovlogique, Chap, VII., 1809; translation in American Naturalist 
for 1888. 
2 Principles of Biology, I1., pp. 167 and 195. 
3 Proceedings of the American Philosophical Society, p. 259; Origin of the Fittest, 
1887, p. 210. 
* Penn Monthly Magazine ; Origin of the Fittest, 1887, p. 30. 
° American Naturalist, 1877, p. 607, 
