No. 381. A HALF-CENTURY OF EVOLUTION. 621 
3 
and not readily appreciable causes are now brought out clearly 
by a system of varied and careful quantitative measurements. 
6. More attention than formerly is given to the study of 
dynamical evolution, or kinetogenesis; to the effect of external 
stimuli, such as intermittent pressure, mechanical stresses and 
tensions by the muscles, etc., on hard parts. Originally sug- 
gested by Herbert Spencer, that the ultimate cause or mechani- 
cal genesis of the segmentation of the vertebrate skeleton was 
due to transverse strains, the segmentation of the bodies of 
worms and arthropods, as well as of vertebrates, has been dis- 
cussed by recent workers (Ryder, Cope, Meyer, Tornier, Hirsch, 
and others). Here should be mentioned the work done in 
general physiology, or morphogenesis, by Verworm, Davenport, 
and others. Also the discoveries of Pasteur, and the applica- 
tion by Metschnikoff and of Kowalevsky of phagocytosis to 
the destruction and renewal of tissues during metamorphosis, 
bear closely on evolutional problems. 
7. A new field of research founded by Semper, Vilmorin, 
and Plateau, and carried on by De Varigny, is that of experi- 
mental evolution, involving the effects of artificial changes of 
the medium, including temperature, food, variation in the 
volume of water and of air, absence of exercise, movement, 
etc. Also should be added horticultural experiments which 
have been practised for many years, as well as the results of 
acclimatization. 
Here should be mentioned the experiments bearing on the 
mechanics of development (Entwickelungsmechanik der Organ- 
ismen), or experimental embryology, of Oscar Hertwig, Roux, 
Driesch, Morgan, and others, and the curious results of animal 
grafting and of mutilations of the embryos, obtained by Born 
and others, as well as the regeneration of parts. The remark- 
able facts of adaptation to new and unfavorable conditions 
of certain embryos are as yet unexplained, and have led to 
considerable discussion and research. 
8. The a priori speculations of Darwin, Galton, Spencer, 
Jaeger, Nusbaum, Weismann, and others, based on the results 
of the labors of morphologists and cytologists, have laid the 
foundation for a theory of the physical basis of heredity, and 
