No. 376.] REVIEWS OF RECENT LITERATURE. 273 
two basal principles of Heredity and Variation, this principle 
of Isolation constitutes the third pillar of a tripod on which is 
reared the whole superstructure of organic evolution.” Natural 
Selection is regarded as merely a special case of isolation when the 
best fitted are separated from the less fit by the death of the latter. 
The following chapters are concerned with another special case of 
isolation, namely, Physiological Selection. Here are brought together 
in convenient form the chief facts and arguments in regard to this 
important subject. This is followed by a chapter giving a brief. 
history of the opinions on isolation as a factor of organic evolution, 
and the general conclusions of the whole work are summed up in 
a final chapter. 
The book is largely controversial in tone, and the arguments are 
presented, of course, from the standpoint of the well-known views of 
the author. Still, the other side is at least given a hearing, and we 
have in the three volumes as a whole what we have not had before, — 
a complete work on organic evolution reflecting the thought of recent 
times. 
ZOOLOGY. 
A Viviparous Holothurian. — The life histories of the few holo- 
thurians that protect their young during the early stages of their 
development afford some most peculiar and interesting instances of 
adaptation, as regards both structure and habits. 
The transference of the eggs in Cucumaria planci to an atrium in 
front of the mouth and encircled by the tentacles, there to be fertilized 
by spermatozoa thrown out by a neighboring male into the surround- 
ing water, which is swept into the atrium by the movements of the 
tentacles of the female, is well known from Selenka’s account of the 
process. 
We are also acquainted with the conditions in Cucumaria glacialis 
Ljungman in which Mortensen! describes a pair of broad pouches, 
invaginations of the body wall in the ventral interradii, immediately 
behind the circle of tentacles. The large yolk-filled eggs (diam. 
1 mm.) after being laid are presumably taken up by the female 
from the sea bottom into the pouches. Similar brood sacs are found 
in another Arctic species, Cucumaria minuta Fabr., and in C. laevigata 
Verr. of the Antarctic seas. 
Th. Mortensen, Zur -Anatomie und Entwicklung der Cucumaria glacialis — 
ica Zeit. f. wiss. Zool., Bd. lvii, pp. 704-732, Taf. 31, 32. 1893. 
