94 



organisms ; dry air induces the formation of a 

 thick cuticle and much skeletal tissue in many 

 plants, while excess of moisture is accompanied 

 by the formation of little cuticle and absence of 

 strengthening tissue ; the presence or absence of 

 water has also a marked effect on the mode of develop- 

 ment of some amphibious organisms (see also p. 98). 

 The Axolotl of the Mexican lakes, for instance, is 

 at one stage aquatic and provided with gills, but 

 develops lungs, like a salamander, when subjected to 

 dry conditions. The effect of artificially altering 

 the salinity of water on the movements and forms 

 of organisms inhabiting it have led to important 

 conclusions on the origin of fresh-water from salt- 

 water faunas. Indeed, the fluids of the body also 

 have been shown to become altered by changed con- 

 ditions of the medium, affecting, as it would appear, 

 the character of the blood corpuscles, the amount 

 of pigment developed, &c., while ciliated cells may 

 be made to become amoeboid and vice versa by 

 varied changes in the medium. It has been found 

 that certain chemicals can induce the unfertilised 

 eggs of certain animals to segment, but the classic 

 researches of the Hertwigs and of Loeb on the fer- 

 tilisation and segmentation of the ovum under 

 different conditions can only be referred to in this 

 connection space forbids their quotation in detail. 

 Pre-eminently favourable nutritive conditions have 

 been found to induce ciliated lower forms to become 

 amoeboid or even to take on cell walls, and it is well 

 known that asexual reproduction by purely vegeta- 

 tive methods is encouraged by such conditions, while 

 vigorous pruning of shoot or root tends to the develop- 

 ment of flowers and fruit. More than one authority 

 claims to have shown that better nutrition tends to 



