On Certain Factors of Evolution. 811 
2. Disuse of certain organs. 
3. Adaptation, enabling the more plastic forms to survive and 
perpetuate their stock. 
4. Isolation, preventing intercrossing’ with out-of-door forms, 
thus insuring the permanency of the new varieties, species, or 
genera. 
5. Heredity, operating to secure for the future the permanence 
of the newly originated forms as long as the physical conditions 
remain the same. 
Natural selection perhaps expresses the total result of the working 
of these five factors rather than being an efficient cause in itself, or 
at least constitutes the last term ina series of causes. Hence 
Lamarckism in a modern form, or, as we have termed it, Neo- 
lamarckism, seems to us to be nearer the truth than Darwinism 
proper or “ natural selection.” 
The factors of organic evolution such as we have mentioned are, 
of course, theoretical, and the critic or unbeliever in a theory of 
descent demands facts in demonstration of the truth of the deriva- 
tion of cave animals. Of the facts we have ourselves observed, or 
which have been observed by others, we will briefly summarize :— 
1. The variations in Pseudotremia cavernarum and Tomocerus 
eet found living near the entrance of caves in partial day- 
ight. 
2. The bleaching of Polydesmus and Machilis found living in 
small caves; the blindness of Neotoma, or the wood-rat of Mam- 
moth Cave; of fish found in wells and subterranean streams; the 
atrophy of the mole’s optic nerves induced in one generation. 
3. The larger size of the eyes of the young than in the adult 
Troglocaris of Europe, and the blind crayfish of American caves; 
Semper’s history of the atrophy of eyes in the parasitic Pinnothe- 
res; eyes of Gammarus pulex affected after living in darkness; the 
eyes of Gammaride in Lake Baikal becoming smaller the deeper 
they live; the instability in the eyes of Cæcidotæa. 
Tn a small cave near White’s Cave, and at a point about sixty 
feet from the mouth, occurred a salamander (Spelerpes longicauda- 
tus Green), which was apparently bleached, being nearly white, with 
dark brown blotches. The common Cambarus bartonii occurs some- 
what bleached in Mammoth Cave, and this may not be the result 
