256 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



changed conditions; hence this delicacy of adjustment is far more 

 necessary in the higher forms of animal hfe 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, therefore, 

 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 



