516 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
derm becomes endoderm because it is invaginated, and not vice versa. 
There is no alternative between epigenesis and the Gemmaria-theory of 
Haacke. Dricsch’s last book is criticised at some length, and then the 
author returns to the inheritance of acquired characters. It does not 
follow that this is impossible because its mechanism is not at present 
explicable, though the Gemmaria-theory makes the process conceivable. 
Direct proof is not forthcoming, and may be impossible, but indirect 
evidence is cumulative. On this, on amphimixis, and other problems, 
Haacke has much to say which is worthy of consideration. 
The Essence of the Organism.* — Dr. W. Haacke criticises at great 
length an essay by Dr. Hans Driesch entitled c Die Biologie als selb- 
standige Grundwissenschaft ’ ; and in so doing expounds his own views 
as to the essence of the organism. We cannot within our present limits 
see our way to a summary which would be just to either party, and 
simply notice the two complementary contributions to the philosophy of 
biology. 
Phytogeny of an Acquired Characteristic.f — Prof. A. Hyatt has 
added a contribution of great length to a subject of which the literature 
is already more than voluminous. He commences with pointing out that 
the nature of the evidence afforded by fossil shells is, even at the present 
time, very little understood. He urges that a single shell, either of a 
living or a fossil form, may present accurately the general history of 
the development of the young, the stages of the adult, and old age. The 
results of heredity and of the action of endemic or traumatic diseases 
may also be detected, if one knows how to study and compare the 
remarkable and distinct series of metamorphoses displayed by this 
external or protective skeleton with those of congeneric forms. Limiting 
himself to the Cephalopodous Mollusca and using the terminology with 
which readers of his papers have more or less made themselves acquainted, 
Prof. Hyatt enters into great detail with regard to a number of fossil 
forms. He points out that it is impossible to disprove or prove that a 
character is acquired or genetic, unless it can be followed back to its 
origin. Until this is done one cannot assert positively that it was not 
potentially existent in the embryo and became apparent at the proper 
time in the ontogeny in accordance with genetic law. His final conclu- 
sions are that his results favour the theory of tachvgenesis and diplo- 
genesis, and are opposed to Weismann’s hypothesis of the subdivision of 
the body into two essentially distinct kinds of plasm. 
Extinction of Species.^ — Mr. C. Morris makes an attempt to solve 
the problem of the extinction of the great numbers of giant reptiles 
which disappeared at the close of the Cretaceous epoch, and the similarly 
sudden disappearance of a considerable number of large mammals in the 
early recent period, such as the Mammoth, the American Horse, and the 
Giant Sloths. He thinks that, looking at the matter generally, few if 
any species have ceased to exist in consequence of the direct assaults of 
other animals, for animal hostility, even when unrelenting and effective, 
is never governed by a fixed purpose of destruction. Hostile aggression, 
* Biol. Central bl., xiv. (1894) pp. 626-47, 666-81, 697-718. 
t Proc. Amer. Phil. Soc., xxxii. (1894) [1895] pp. 349-647 (14 pis.). 
j Proc. Acad. Nat. Sci. Philad., 1895, pp. 253-63. 
