ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
691 
by these factors of transformation in sections prevented from crossing 
should be completely parallel in the different sections, even when 
exposed to the same environments. The next step which is taken in the 
latter half of the present paper, is to show “ that the relations to each 
other of varieties, species, genera, and the higher groups are such as 
would necessarily be presented if all such differences were the result of 
evolution that is always dependent on some form of segregation, but 
not always on diversity of natural selection, nor always on exposure to 
different environments.” 
Mr. Gulick maintains and cites facts in support of his view that 
“persistent differences, whether varietal, specific, or generic, are not all 
adaptational, for some of them have no relation to utility ; and that 
adaptational differences are not all advantageous, for some of them relate 
to adaptations that would meet with equal success if the organisms 
should exchange habitats ; but that in every case divergence, whether 
utilitarian or non-utilitarian, whether advantageous or disadvantageous, 
is not maintained without independent generation,” or, in other words, 
requires the prevention of interbreeding. 
The paper contains an interesting discussion of the various (eight) 
factors of transformation and of the different forms of selection, while 
the argument is illustrated by an account of divergent evolution in the 
land-molluscs of Oahu and in a number of insects. But we are not able 
to do more than give the general argument by which Mr. Gulick shows 
how segregation is intensified. 
Definition of Species.* — Prof. F. Lataste offers the following defini- 
tion of a biological species. The category of organized beings designated 
under the name species is composed in space and time : — 
(1) Either of two collections of individuals of different sexes, one of 
which — the female — is generally capable of being fecundated by a male, 
or of a single collection of non- sexual or hermaphrodite individuals ; the 
individuals of each collection, on the one side, present with one another 
the same relations of resemblance as with all the normal descendants of 
any one of them (general case), or at least with some of the descendants 
(cases of polymorphism and alternation of generation) ; and, on the 
other side, the morphological interval between the most distinct indi- 
viduals of each collection is filled up by intermediate individuals in 
such a way that we pass from one to the other by an insensibly 
gradual series of modifications. 
(2) And of all the individuals which certainly arise from those 
comprised either in the single or in the two collections, as above defined, 
whatever be the degree in which they may differ, either normally and 
collectively (polymorphism, alternation of generation), or accidentally 
and individually (monstrosities). 
Comparison of Oogenesis and Spermatogenesis.f — Prof. 0. Hertwig 
makes a fresh comparison of oogenesis and spermatogenesis in Nematodes, 
and finds in this “a basis for the discussion of disputed questions.” 
The mother-sperm-cell corresponds to the immature ovum. During a 
prolonged resting-state the nuclear substance in both is peculiarly pre- 
* Zool. Anzeig., xiii. (1890) pp. 480-3. 
f Arch. f. Mikr. Anat., xxxvi. (1890) pp. 1-138 (4 pis.). 
