No. 376.] REVIEWS OF RECENT LITERATURE, 283 
temperature, annual distribution of crustacea, with a discussion of 
the factors controlling annual distribution, vertical distribution, and 
a discussion of the factors controlling vertical distribution. 
In treating of temperature, Professor Birge proposes the term 
thermocline as the equivalent of the German Sfrungschicht, a happily 
chosen word, which will, doubtless, have a permanent place in 
scientific nomenclature. The thermocline is that layer of water in 
which there is a sudden change from the warm temperature of the 
surface to the cooler temperature of the lower waters. The thermo- 
cline lies at a depth of from eight to ten meters, and at this point’ 
there may be a change of as much as nine degrees in a single meter. 
The position of the thermocline is somewhat variable, being deter- 
mined not simply by convection, as is intimated by most German 
authors, but also by the effect of the wind. It follows that the 
thermocline will be higher in a lake protected from the winds than 
in a larger body of water. 
The author then gives a detailed account of the annual distribu- 
tion of the forms of crustacea found in Lake Mendota. His results 
are not entirely in accord with the observations of European authori- 
ties or with those of the reviewer in Green Lake. It is perhaps 
remarkable that the agreement should be as great as it is, when we 
consider the differences in environment of the lakes. ) 
The factors of annual distribution are stated to be food, tempera- 
ture, and competition. It follows because of variations in the amount 
of food and the yearly changes of temperature, with a general corre- 
spondence between successive years, that there are yet marked 
differences. He finds evidence of competition in the fact that certain 
species are predominant at times to the exclusion of others, this 
being explained on the assumption that the water can support only a 
certain number, and that the first species that gains control of the 
field will inevitably exclude others. 
In speaking of the vertical distribution of crustacea, he records 
observations on the migration in the upper meter, finding evidence 
of a very distinct diurnal migration within narrow limits. He finds, 
too, that the thermocline seems to be the lower limit of the vast 
majority of crustacea, only a few forms being found in abundance 
below that point. 
In the account of the vertical distribution of individual species 
there are many interesting facts. He finds the distribution of 
Diaptomus very different from that reported of the Diaptomi of 
Green Lake. This does not seem to me strange, for the Diaptomus 
