ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
173 
cautious, but is tlio caso really so bad ? Haackc lias no hopo of our 
ever finding the parent stocks of tlio great types, his interest is in the 
possible physiological factors in the evolution. 
“ Constitutional selection ” leads to increase of size, to differentia- 
tion. An ancestral blastula becomes invaginated and variously folded. 
Folding is the great process of differentiation, but he does not tell us 
why the folding occurs, except that it is due to “ constitutional selec- 
tion.” The increased temperature of Mammals is referred to the 
increase of the lung surface by folding, which would have especially 
marked effects in a body so small as that of the early Mammals seems to 
have been. Hairs arise from the stimulus of the hot blood in the skin, 
and a case is cited where an unwonted difference of temperature between 
skin and outer world was associated with strong growth of hair. 
Perhaps the increased temperature of the blood was also associated with 
the origin of sebaceous glands, which probably arose in correlation with 
hairs. 
It is startling to find it suggested that brood-pouch and mammary 
glands were more likely to have been acquired by the males than by tho 
females. The early Mammals were possibly squirrel-like in their pose, 
and the males are supposed to have carried the eggs in their lap, as it 
were, till folds and a pouch naturally arose. By-and-by the males tired 
of this little way of theirs, but not before the presence of the eggs 
and young in the pouch had stimulated the integumentary glands into 
milk-giving. This acquisition has also been handed over to the females, 
though it is interesting to notice that the mammary glands of the male 
Echidna are exceedingly large. Moreover, Haacke does not exclude tho 
possibility that when the Waitoteke of New Zealand is found it will be 
a male of the old-fashioned sort. 
Dr. Haacke knows as well as any one else how bold some of his 
speculations are, and submits that they can do no harm as long as their 
purely speculative character is recognized. Though he solves no 
problems, his vigorous essay is stimulating, and it is refreshing to read 
such trustful Lamarckianism. 
Dentition of Mammals.* — Prof. W. Leche adds to his recent im- 
portant essay on the development of the dentition in Mammals an 
appendix dealing with Myrmecobius fasciatus, four types of Chiroptera, 
and Phoca grcenlandica. In young forms of Myrmecobius he finds traces 
of a dentition acquired by lower Vertebrates, and preceding the first or 
milk-dentition of placental Mammals. As to the bats, he notes the 
peculiarity of Desmodus rufus in having the most anterior deciduous 
premolar laid down but not developed. Divergent as is the adult denti- 
tion of Desmodus , the milk-dentition agrees generally with that of the 
related Stenoderma. Important is the great independence of the 
enamel ridge, as seen in the very complete constriction off of the enamel 
germ, and in the occurrence of a swollen end of the ridge beside the 
premolars. In this, as also in Phoca , Leche finds evidence of a pre- 
disposition towards, or possibility of, a third set of teeth. After 
describing some stages in the dental development of P. grcenlandica , the 
author discusses certain general questions. 
* Morphol. Jakrb., xx. (1893) pp. 113-42 (12 figs.). 
