256 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
changed conditions; hence this delicacy of adjustment is far more 
necessary in the higher forms of animal life than in more stationary 
plant organisms, and in the developing nervous systems of animals we 
have just the central adjusting system that is required for these condi- 
tions. With evolution of type there will thus be an increasingly definite 
tendency given to organic, especially the animal, forms of life, if the 
acting principle of evolution has been selectional. Selection is, therefo-e, 
able to account for the steadily progressive tendency of life as a whole 
without calling to its aid any unknown and doubtful perfecting principle. 
To summarise: Natural selection, acting on the whole organism, 
tends to produce more and more definite tendencies in all surviving 
forms of life, which tendencies are progressive and continuous in char- 
acter. Variable conditions, by partially altering the line of selection, 
induce a temporary indefiniteness. And lastly, the process of selec- 
tion being itself able to be the indirect, though not the direct, cause 
of those favourable variations, which it subsequently selects from, is 
able to dispense with any subsidiary factors, provided it has a certain 
number of elementary properties of life which afford sufficient material 
to work with. 
EXPERIMENTAL SUPPORT OF THE EFFECTIVENESS 
OF NATURAL SELECTION 
Weldon’s experiments with the shore-crabs of Plymouth Sound.— 
These experiments seem to show that under changed environmental 
conditions natural selection acts upon minute fluctuating variations 
of linear or quantitative type so as to produce an alteration in the 
species; exactly as Darwinism would hold. A large breakwater was 
so placed near the mouth of Plymouth Sound that the rate of flow of 
the river water was greatly slowed down in certain regions. This 
allowed an increased settling of the fine china-clay sediment that is 
carried by the river, and the changed condition caused the death of 
numerous crabs of the species Carcinus maenas. The question arose 
as to whether the survivors and those that had perished showed any 
consistent differences on the basis of which selection could be operat- 
ing. Careful measurements of hundreds of individuals showed that 
the mean breadth of frontum is slightly less in the survivors than in 
the perished. Measurements were repeated in two subsequent years 
and it was found that there was a progressive narrowing of the 
frontum. As an experimental check upon these conclusions Weldon 
