790 The American Naturalist. [September, 
lished in full his paleentological evidence that an impression 
in the shell of fossil Ammonites, due to crowding of coils, per- 
sists in (abnormally) uncoiled species. A masterly discussion 
of the whole question of inheritance of acquired characters 
appeared this year in Romanes’ “ Post-Darwinian Questions.” 
Variation —If little new was added to our knowledge of 
heredity, such was by no means the case with variation. New 
facts were acquired, new methods of study employed, new ex- 
perimental investigations made to determine its cause. 
Osborn classified variations as ontogenetic (and either gona- 
genic, gamogenic, embryonic or somatogenic) and phylogen- 
etic. Scott distinguished between individual variation (of on- 
togenetic value only) and mutation (of phylogenetic value). 
Mehnert showed that variation occurs as abundantly in em- 
bryos as in adults. Eigenmann showed that in certain fishes 
the variants above the mode are more abundant than those 
below, and that individual variation is greatest where the 
number of species is greatest. The extraordinary variation of 
meduse was investigated by Browne. 
The development of the mathematical study of evolution, 
for which this decade will ever be famous, took a great stride 
in the publication of Pearson’s “Skew Variation,” by whic 
methods of measuring unsymmetrical variation curves, their 
variability and their skewness were given. Since most bio- 
logical curves are skew curves, this method greatly extends 
Galton’s, which was applicable only to symmetrical curves- 
DeVries studied quantitatively a case of dimorphism in plants, 
and Weldon investigated selection in crabs. 
Among the studies on the causes of variation may be men- 
tioned the experiments of Vernon on echinoderm larv®; of 
Weismann, Standfuss, Ris and, especially, Fischer (similar 
effect on heat and cold), upon lepidoptera; of Bonnier on 
plants subjected to electric light (producing excess of chloro- 
phyll and scragged form); and of Goebel, who found that 
when cacti with foliaceous stems were grown in the dark the 
stems became rounded. Davenport and Castle found that 
tadpoles have the capacity for self-adaptation to heat. 
