1886] Recent Literature. 361 
rely on purely geological causes ; they may yet be proved to have 
been sufficient. Dr. Croll nowhere explains why so large a por- 
tion of the subpolar regions were unglaciated. 
In his discussions in cosmology, Dr. Croll contends that Sir 
William Thomson and others are wrong in maintaining the 
“gravitation theory,” z. e., that the sun cannot have supplied the 
earth with heat, even at the present rate, for more than about 
15 to 20,000,000 years. He discards this theory, and freely gives 
the evolutionists and geologists all the heat they want, by claim- 
ing that the sun’s heat was originally derived from motion in 
space; this being “ more in harmony with the principles of evolu- 
tion than the gravitation theory, because it explains how the 
enormous amount of energy which is being dissipated into stellar 
space may have existed in the matter composing the sun untrans- 
formed during bygone ages, or, in fact, for as far back as the 
matter itself existed.” 
On page 65, Dr. Croll, it seems to us, too hastily assumes that 
the ice in the interior of Greenland is of great thickness, while 
the land itself is low, “ probably not much above sea-level.” On 
the contrary, as the result of recent Danish exploration, Dr. Rink 
tells us, the surface of the ice in the interior is 6000 feet above 
the sea, while we infer from his statements that the thickness of 
the ice is not much over 2000 feet. In fact, the theoretical gla- 
Cialists go to extremes ; closet speculations and field-work do not 
always harmonize. 
The only typographical errors we have noticed are the mention 
of “Heyes” for Hayes on one occasion, while Torell is wrongly 
spelt “Torrell,” in the only instance in which it is used. 
Leunis’ SYNOPSIS DER THIERKUNDE.'—This is a new edition of 
Leunis’ Zoology, which for so many years has been in almost 
universal use in the German gymnasia and many of the universi- 
The present work contains two large volumes of more than 
1200 closely printed pages and 1000 cuts each. Perhaps the 
_ the student analyzes flowers with Gray’s Botany. This alone 
Would render the book invaluable to any one who wishes to 
in the study of a new group or to determine quickly an ani- 
Seas. But it contains representative species of most of our Ameri- 
can genera. Professor Ludwig, whose work on Echinoderms is © 
n by all zodlogists, has revised the edition and has com- 
Pletely rewritten the second volume, which treats of the inverte- 
LTH; oe 
Third edition, revised by Professor Lupwic, of Giessen. 
