240 SCIENCE PROGRESS. 



to other conditions than that of temperature, yet the proba- 

 bility is that such is the case, and that conditions such as 

 the saHnity of the water also have a more powerful in- 

 fluence. 



Another conclusion arrived at, which is of some im- 

 portance in the study of the problems of variation, is that 

 changes of environment may produce different and opposite 

 effects upon different parts of the same organism. Thus a 

 fall in the temperature or a decrease in the salinity of the 

 water produces a decrease in the arm lengths of the larvae 

 and an increase in the body length. It is thus possible for 

 parts of an organism to become modified, though they may 

 be entirely unacted upon by the process of Natural Selec- 

 tion, or may even serve some useful purpose to the animal. 

 If, for instance, it is of greater utility to the larvae that 

 their body lengths should increase than that their arm 

 lengths should increase, and if by the action of a fall in the 

 temperature of the water larvae with greater body lengths 

 and smaller arm lengths are produced, then, on an average, 

 the larvae exhibiting these changed characteristics to the 

 most marked extent will survive, and the race will be modi- 

 fied in this direction. 



BIBLIOGRAPHY. 



(i) Darwin, T/ie Origin of Species, sixth ed., p. io6. 



(2) Montgomery, T. H. Organic Variation as a Criterion of 



Development, MorpJi. Joiirn., p. 251, 1896. 



(3) Wallace, A. R. Darwinism, p. 413. 



(4) Welsmann. The Germ Plasm, pp. 413, 431 and 454. 



(5) Weldon, W. F. R. The Variations occurring in certain De- 



capod Crustacea, /. Crangon vulgaris. Proc. Roy. Soc., xlvii., 

 445. Also, Certain Correlated Variations in Crangon vul- 

 garis. Proc. Roy. Soc, H., 2. 



(6) Vernon, H. M. The Effect of Environment on the Develop- 



ment of Echinoderm Larvai : an Experimental Inquiry into 

 the Causes of Variation. Phil. Trans., pp. 577-632, 1895. 



H. M. Vernon. 



