448 THE ZOOLOGIST. 



colourless, have a broad tail-fin, and have three pairs of gills 

 instead of one, these being already adapted for aquatic respira- 

 tion when the larva hatches. 



Sexually mature males in the third generation have rough 

 callosities on the ball of the thumb, and in the fourth generation 

 these are coloured black. In addition these males have hyper- 

 trophy of the muscles of the fore-arm. These two effects are 

 evidently correlated with the extra difficulty experienced by the 

 male in holding the female during the copulatory process, owing 

 to her skin being wet and slippery in the water. 



To return to the eggs laid on land and abandoned by the 

 male. If they are kept on damp, cool soil they develop nor- 

 mally, but if the temperature is kept high they develop more 

 rapidly, and if they are prevented from hatching at the proper 

 time by only giving them the minimum amount of moisture 

 very large eggs are obtained, in which the embryos remain for 

 six or seven weeks, and when hatched are already 20 mm. long. 

 If they are also kept in a feeble light giant eggs are formed in 

 which the embryos remain for ten weeks, reaching a length of 

 31 mm., and possessing the hind pair of legs. The remaining 

 process of development only takes about five months, and the 

 adults produced are distinguished by their dwarf size. These 

 dwarf Toads only produce sixteen to nineteen eggs, from which, 

 if the experimental conditions are discontinued, larvas emerge 

 after about seven weeks. They are 21 mm. in length, and have 

 undifferentiated stumps as rudiments of the hind limbs. If the 

 experimental conditions are continued, the larvas hatch out at an 

 even more advanced stage than in the previous generation. 



Larvae from the original land-eggs, or from those in which 

 the embryos have been retained for an extra long period, can 

 live several weeks on damp earth instead of in the water. Their 

 integument becomes thickened, and they develop epidermal 

 glands and lungs ; also the tail-fin diminishes, and the muscles 

 of the limbs strengthened. They have to be placed in the water 

 to undergo the metamorphosis, but this takes place in a very 

 short time, the resulting adults being very much dwarfed. If 

 the conditions are relaxed in the next generation, these induced 

 modifications are almost lost, though the lungs are better deve- 

 loped than usual, and metamorphosis occurs earlier. If the 



