594 
element in the evolution of specific types. 
His papers have been useful in putting a 
needed emphasis on a factor which had been 
insufficiently taken into account and _ fre- 
quently overlooked by theorists concerned 
with the question of specific evolution. It 
was appropriate, therefore, that the Carnegie 
Institution should give him the opportunity 
of presenting in one handsome volume, the 
ripened result of his years of reflection and 
study on this subject. 
It is known that his studies were largely 
due to the interest excited by the beautiful 
and multiform tree-snails of the Hawaiian 
Islands, which, for variety in characteristics 
elsewhere usually taken as of specific value, 
are unexeelled in any equal area. It was a 
problem which appealed to every collector of 
these attractive How should this 
almost infinite variety under almost identical 
conditions be accounted for? The latest in- 
vestigations indicate that the chief food of 
the arboreal Achatinellas consists of fungoid 
mycelium which in the warm air and con- 
animals. 
stant rains of the mountainous region of the 
islands is more or less abundantly developed 
on the bark of trees and shrubs upon which 
these landshells live; an examination by Mr. 
Cook of many stomachs has shown that the 
leaves of the shrubs or trees form no part of 
their diet, and that, contrary to the opinions 
formerly held and even not altogether dis- 
carded in the volume under review, the species 
of tree upon which these animals live is not 
of importance in their economy; the same 
species of shell being often found indiffer- 
ently upon different species of trees over the 
area the former This fact lends 
even greater importance to the remaining ele- 
inhabits. 
ments of the environment among which the 
stimulus to variation is to be sought. 
It has been found that the Achatinellas do 
not lend themselves readily to experiment. 
Removal, even when not the slightest injury 
has been inflicted, usually proves fatal, from 
It is evident that 
they are extremely sensitive to even minute 
some unexplained cause. 
changes in altitude, moisture, ete., and at- 
tempts to get them to breed in the more ac- 
cessible regions of the islands, where they 
SCIENCE. 
[N.S. Vou. XXII. No. 567. 
could be kept under continuous observation, 
have so far proved failures. Even the eggs 
seem unable to bear transportation. 
For the reader who wishes to gain quickly 
an idea of the hypothesis maintained by Dr. 
Gulick, we should suggest the original papers 
of which a bibliography is given in the pres- 
sent volume, as they contain the meat of the 
matter in more concentrated form. In the 
opinion of the reviewer something has been 
lost by the considerable expansion of verbiage 
to which the statement of the hypothesis has 
here been subjected. But doubtless the special 
student of these recondite problems will find 
the volume none too long. In any event it 
should not be forgotten that while Dr. Gulick’s 
views seem eminently probable and in the re- 
viewer’s mind go far toward accounting for 
many of the facts, nevertheless they are 
theoretical and have not yet been subjected 
to the crucial test of experiment, by which the 
proposed theory in the end must be tested. 
To justify final acceptation an hypothesis 
must not only be capable of accounting for 
the facts but it must be shown to be the only 
one by which they may be adequately ex- 
plained. It .is also necessary to determine 
how far the animals in question have arrived 
at that state of organic equilibrium which we 
recognize by the name of species. If, as has 
been held by some authorities, the small color- 
groups are really only of a temporary nature, 
and lable to immediate change upon subjec- 
tion to modified environment, then the au- 
thor’s hypothesis, while losing nothing of its 
truth, is not a contribution to the evolution 
of species so much as to the physiology of 
color-variation. The latter may or may not 
be, in the group discussed, a factor of specific 
weight. 
In any case we are grateful for the full pres- 
entation of the author’s views which are of 
acknowledged importance in the discussion. 
The volume is well printed, though we could 
have wished that the colored plates had been 
of a better quality. W. H. Dat. 
Marceli Nencki Opera Omnia. Gesammelte 
Arbeiten von Professor M. Nencxri. Braun- 
schweig, Friedrich Vieweg und Sohn. 1905. 
¢ 
