246 THE AMERICAN NATURALIST. [Vor. XXXVI. 
Fish Commission Report for 1900, pp. 185—194). It is well known 
that fish eggs develop much more rapidly in warm water than in 
cold, and when, as is often the case, the temperature of the water 
at a hatching station varies from day to day, it is often difficult to 
estimate the stage of development reached by a given lot of eggs. 
This difficulty can be. overcome by the use of the temperature unit 
system. By a “temperature unit" is meant one degree Fahrenheit 
above 32° F. for one day. Thus 36° F. for a day is equivalent to 
four temperature units. In any lot of eggs the stage of development 
is recorded by adding the temperature units to which they have been 
subjected since fertilization. The success of such a system depends 
upon the uniformity of its results. As judged from the experiments 
on salmon at the government station at Baird, Cal., the method gives 
a close approximation to uniformity. In something over fifty lots of 
salmon eggs, in which the incubation period varied from forty-eight 
to ninety days, the greater number of eggs hatched. at about nine 
hundred temperature units, the extremes being 874 and 940. The 
utility of this method, not only to fish culturists but also to embry- 
ologists, is evident. 
C. E. Beecher has contributed to the series of Yale Bicentennial 
Publications a volume entitled Studies in Evolution (New York, 
Charles Scribner's Sons, 1901). The volume is made up mainly of 
reprints selected from the publications of the Laboratory of Inverte- 
brate Paleontology at Yale, and contains studies on the origin and 
significance of spines, on the structure and development of trilobites, 
and on the development of brachiopods. Most of the papers have 
already appeared in various journals, but their collection into a 
single volume will be welcomed by advanced students, not only as 
evidence of the work done at Yale, but as an indication of the new 
fields into which modern paleontological research is making its way. 
The stony corals of the Porto Rican waters have been reported 
upon by T. W. Vaughan (United States Fish Commission Bulletin Jor 
I900, Vol. II, pp. 289-320, Pls. I-XXXVIII) About twenty-five 
species are recorded and beautifully illustrated by process plates from. 
photographs. | | 
C. W. Hargitt and C. G. Rogers give an account of the Alcyo- 
naria of Porto Rico (United States Fish Commission Bulletin Jor 1900, 
Vol II, pp. 265-287, Pls. I-IV). Some twenty-five species are 
reported, of which five are new. dun "oio 
