53!7 
1882.] Experimentation in Biology. 
thoroughly explored.” These sentences epitomise an ex- 
tensive portion of a literature supported from month to 
month by contributions from a heterogeneous assemblage of 
persons who discover their priority of mission to detedt the 
oversight of Darwinism, whose negation of their interpre- 
tations of that system is presented to them in the very 
works they criticise, in the two passages cited above, to 
which others might be added. Favourably contrasting with 
the afore-mentioned is a distindt class of men who hold 
sufficient trial has not been accorded to fadtors in variation 
specified by other writers, and treated anew by, but con- 
signed to subordinate positions in the system of, Mr. Charles 
Darwin. 
One of these fadtors is known as “ external conditions.” 
Mr. Darwin devotes a chapter to the subjedt in the “ Varia- 
tion under Domestication,” and a paragraph or so in the 
“ Origin of Species.” The subjedt commands a fuller respedt 
in Mr. Herbert Spencer’s system, while more recently 
special attention has been paid to the question, the various 
influences having been classified, and more or less copious 
examples cited under each sedtion, in a volume of which we 
give the full title and a condensed account of such of the 
subjedt-matter as bears upon our question : — “ The Natural 
Conditions of Existence as they affedt Animal Life,” by 
Karl Semper. The influences recognised are food, light, 
temperature, air, and water, still and in motion, living 
organisms, and miscellaneous. Prof. Semper discusses 
largely the influence of natural conditions, which it would 
be useless to recapitulate here ; he likewise records many 
experiments as to the influence of artificial conditions ; but 
these, it happens, are mostly of so trivial a nature that we 
may omit their consideration also. Several experiments 
refer to vertebrate animals, and thus are most intimately^ 
related to our enquiry : these are as follows : — The experi- 
ments of John Hunter and Dr. Holmgren on the trans- 
formations occurring in the coats of the stomach of birds, 
also mentioned by Mr. Darwin ; the enclosure of Amphibia 
in masses of gypsum ; the darker colouring of frog’s skin 
following deficiency of food ; the experiments of Pouchet 
on the chromatic fundtion in fish, proving the condudting 
fundtion of the sympathetic system of nerves — he severed 
the connedtion of certain of these with the spiral nerves, 
and produced a zebra-like marking on one side of the fish, 
the chromatophores of the side in relation to the uninjured 
nerves, which, being intadt, condudted the optical stimulus 
as normally, retaining their natural hues as regulated by the 
VOL. IV. (THIRD SERIES). 2 N 
