THE RAW MATERIALS OF EVOLUTION 121 



nearly related species only because the conditions 

 of their life are difierent. 



Individual Plasticity. — At all events, there 

 is great interest in individual plasticity, in what 

 can be effected by changes in nurture. We must 

 picture the living creature as continually running 

 the gauntlet of the mechanical, chemical, physical, 

 and even animate influences that make up its 

 environment. It passes over a series of anvils, 

 on each of which the hammers ring a different 

 time. Let us take a few illustrations from among 

 the many. 



If the alkalinity of the sea-water be sUghtly 

 altered, the egg of a sea-urchin allows itself to be 

 fertilised by the sperm of a starfish, or of a crinoid, 

 or of a mollusc (!), producing larvae which all take 

 after the mother. 



If the chemical and physical state of the sea- 

 water be sUghtly disturbed, artificial partheno- 

 genesis can be induced in starfish and sea-urchin, 

 in worm and mollusc. 



Sometimes the result of a slight chemical change 

 is very perplexing, and there are many experiments 

 at which we look with bated breath. Quaint 

 abnormal larvae of sea-urchin, and frog are obtained 

 by adding a little lithium to the water, and the 

 addition of a little magnesium to the sea-water 

 containing embryos of the fish Fundulus hetero- 

 clitus induces in a large number of these the de- 

 velopment of a single Cyclopean eye in place of 

 the normal two eyes.* 



A small Crustacean called Gammarus, very 

 common in fresh water, has the habit of avoiding 



1 (Dharles R. Stoekard, in Journal of Experimental Zoology, 

 february 1909. 



