1897.] The Advance of Biology in 1895. 791 
As for variation due to internal causes, Meyer determined 
that despite its fewer chromosomes, Ascaris wnivaleus is as var- 
iable as A. bivalens, which is opposed to Weismann’s theory. 
Brooks pointed out apropos of amphimixia that the number of 
ancestors of an individual does not roll up according to the 
formula 2n (in which the power » represents the number of 
generations) because of constant intercrossing of relatives. 
Origin of species—A trend towards facts is clearly discern- 
able in the work of the year on this subject. Natural selection 
was tested by the statistical method (Weldon). Galton called 
for facts concerning sports and their pedigree, a call which 
should not be unheeded by American naturalists. One such 
case, excellently traced, was given in 1895 by Tracy in the 
American Natrurauist. Aerial discussions still went on, how- 
ever. Wallace still thought that specific differences arise by 
the summation of slight variations, and Henslow still main- 
tained the view that they arise from considerable self-adaptive 
changes. 
Mental functions.—The differentiation of comparative physio- 
logical psychology from “metaphysics” made good progress 
during 1895. Lloyd Morgan did much to give to instinct a 
satisfactory biological definition. Among special works on 
the senses of animals may be mentioned the Peckhams’ obser- 
vations, showing that spiders recognize each other by sight, 
and Riley’s experiments with moths, in which a marked male 
found a female a mile anda half away. Hodge and Aikins 
gave the records of the activities of a single Vorticella observed 
during several consecutive hours. 
Studies in the ontogenesis of mental functions were made by 
Mills on the dog, and he and Lui agreed that there is a close 
parallel between the appearance of certain functions and the 
visible development of corresponding cortical centres. Bald- 
Win had followed carefully the mental development of a child 
and laid great stress upon the rôle of imitation in the process. 
The development of memory, especially visual memory, and 
the formation of abstract concepts were also studied. 
_ General theories.—This year was productive of no new guid- 
ing theories. It was still reasserted that it is vain to seek 
