THE PLASTICITY OF LIFE 203 



gradually upwards, as if it had been pruned with a knife. 

 The same sort of thing is seen in passive animals, such as 

 sponges and corals. Of particular interest are cases where 

 the same creature assumes quite different forms in different 

 surroundings : thus it seems well proved that the huge 

 Neptune's cup (Poteriori) is the free form of a little shell- 

 boring Clionid. Sometimes the environmental influences 

 are hard to separate from influences of function : thus 

 Camerano observed that tadpoles of Rana muta in swift 

 streams had longer and larger tails than those in quiet 

 pools near by. 



A second set of environmental factors may be summed 

 up under the general title, chemical, including the chemical 

 composition of the surrounding air, water, soil, or other 

 medium, including also the quality and quantity of the 

 food. It may be fairly said that we are just beginning to 

 appreciate something of the subtlety of relations between 

 marine animals and the variable medium of sea-water in 

 which they live. 



By altering the salinity of the water, Schmankewitsch 

 changed one variety of brine-shrimp (Artemia salina), in 

 the course of generations, into another variety, and 

 then reversed the process. It is very difficult to tell in 

 many cases how far the structural change is the direct 

 result of the environmental change, or how far the latter 

 is only the liberating stimulus of the former setting free 

 some potentiality that was lying unexpressed in the egg 

 or young creature. Let us take a few illustrations col- 

 lected elsewhere. 



" If the alkalinity of the sea-water be slightly 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 slightly 



