818 On Certain Factors of Evolution. 
ness. We have in the atrophy of the optic nerves of the mole a 
parallel case in the blind Myriapod Pseudotremia cavernarum, 
where the eyes survive but the optic nerve is wanting, as also in a 
less marked degree in some of the individuals of Cæcidotæa stygia. 
The study of the conditions of existence in caves is of special 
value, because such conditions are so unusual and abnormal and the 
results upon certain organs so easily appreciated. It is by a study 
of life under unusual conditions that the attention is aroused and 
interest is excited, and after acquiring experience in dealing with 
the more palpable, because somewhat abnormal, circumstances under 
which organisms exist, we can then more easily observe the effects 
of changes of ordinary conditions upon the organism. 
From a study of cave life, of organisms existing in saline and in 
heated waters, of plants and animals. exposed to great cold in alpine 
or polar regions, of those living in hot, dry deserts, we can turn to 
an examination of the results of adaptation to a parasitic mode of 
life. The strange modifications of form, owing to disuse, in internal 
as well as external parasites of different orders and classes, the 
change of host necessitated, and the intensity of the struggle for 
existence in animals living under such exceptional conditions, 
embryology proving that they have arisen from animals of normal 
organization,—such studies as these are of fundamental importance 
in a discussion of the origin of species and higher categories. 
Moreover, the study of the results of the incoming and cessation of 
the Glacial epoch, the effects on life arising from the elevation and 
depression of the land, involving not only change of land surfaces, 
but a change of climate,—it is by a study of such marked changes 
as these in the conditions of life that we are prepared to examine 
the more subtle causes of variation throughout the organic world 
in general, 
After the foregoing pages were written we read with much 
interest Mr. Herbert Spencer’s recent essays entitled “The Factors 
of Organic Evolution.”! While that author, it appears to Us, lays 
too great stress on Dr. Erasmus Darwin’s views, as compared with 
Lamarck’s ; the author of the Philosophie Zoologique having been 
a professional botanist and zoologist as well as a naturalist of the 
1 New York, 1887, reprinted from the Nineteenth Century for April 
and May, 1886, 
