Finally, our knowledge of the physiology of nerves and ganglia, as well as of the brain. of the more specialized animals and of man, has had its bearings on comparative psychology, and under the influence of the experiments of Loeb. Billie, and others we are coming into a position to dis criminate between simple physiological or reflex acts and instinctive acts, and to compare them with the nets of human reason. And here again we see that plants and animals approximate, in that the reflex acts of the lowest animals, those without nerves or ganglia, are akin to the re stricted movements of certain plants.
As is well known, Darwin's theory of selection is based on his assumption of the universal ten dency to variation, a phenomenon he did not at the outset attempt to explain. The Lamarekian factors of change of the conditions of life, of climate, and soil, are what we rely on I o aeomnt for variations, as indicated by Darwin later in life, strongly insisted on by Herbert Spencer, by Simper. Kerner, and their suceessors. Within the past few years the study of varia tion, from a general as well as statistieal point of view, has already reached very considerable proportions. According to the results obtained
by J. A. Allen, Wagner, Wallace, and others, a very large proportion of existing varieties and species are local, geographical, or climatic forms, due to migration, geographical changes, and re sulting isolation. Bateson has collected in a thick volume all the known eases of discontinu ous variations, and many species, as claimed by Galion, also by De Vries, owe their origin to sports or aberrations. Statistical variations, consisting of the minute measurements of multi tudes of individuals, studies initiated by Gould, Baird, Allen, Weldon, Pearson, Davenport, and others, all tend to show that the variations are due to local differences in climate, soil, tempera ture, the nature of the medium, etc., with the result that the doctrine of the essential unfixity of species, the high degree of unstable eqnilib- • limn of organisms, is a fundamental fact with which the biologist has more and more to deal.
Consult: Gesehichte der Zoologic (Munich, 1872) ; Packard, in Standard Natural History (1885) ; Packer and Haswell, Zoology (New York, 1897).