NOTES. 429 



fordia and Brmicliijnts stagnalis and tmrnooriiii may be exposed to 

 the titmost heat of the sun -without perishing. 



Note is, page 121. Summer-sleepers are found among the most dis- 

 similar groups of animals. The Tenrec of Madagascar (^Centetea) is 

 well known. Darwin found insects, spiders, sneiils, toads, and lizards, in 

 a summer-sleep, in Brazil. Most of the land mollnsca of the Mediter- 

 ranean province pass the summer in a dormant condition, and the same 

 is true in the tropics. (Comp. Schmarda, Tliiergeog. i. 12.) As I have 

 said in the text, in most oases, perhaps in all, the true cause is the 

 dryness of the air which is usually associated with a high tempera- 

 ture. Uo experiments have been made that can prove that dnriug the 

 summer-sleep the vital processes are not merely reduced to a minimum 

 of energy, but also altered as to their nature — as they are in winter- 

 sleep. 



Nate 44, page 122. It is known that in man for instance, sexual 

 maturity is attained at a much earlier age between the tropics than in 

 northern climates ; girls of twelve are in Cuba regarded as fully grown 

 and marriageable. This phenomenon is far more striking in swine. I 

 myself have seen pigs in Manila of which the males at three weeks old 

 were ready, and fit to be put with fully grown females. But the great 

 variety of circumstances which co-operate to produce such early maturity 

 leave it doubtful whether it is solely due, as is assumed, to the high 

 temperature of tropical climates. We have seen, in note 15 to Chapter 

 II., an instance of early maturity induced by suitable nutrition. 



Nate 45, page 125. It is known that during the winter frogs eat 

 little and hardly grow at all, nevertheless their eggs are formed during 

 that season. Precisely the same obtains with regard to Lymnma; I 

 have proved that the minimum of temperature which allows them to 

 assimilate food and so to grow is much above the winterTtemperature 

 at which they deposit their eggs. It is known, moreover, that the larvie 

 of frogs which have not grown fast enough in the first year to allow of 

 their transformation taking place in due time, live through the winter 

 as tadpoles and do not begin to grow again till the following summer. 

 Now, it would be interesting to investigate whether in larvse thus 

 retarded by cold the germ-glands are any further developed than is 

 normally the case in the larva stage. 



Nate i6,page 125. According to the result of researches conducted 

 by me during many years, in large individuals of the Axolotl about 

 48 hours are sufficient to allow of a large number of unfertilised ova to 

 pass from the ovary into the oviduct, to become surrounded with an 

 albuminous envelope, to be fertilised and deposited. I have repeated 

 the experiments which prove this many successive years with the same 

 results. If the Axolotl is kept in small aquaria without plants or 

 sand, individuals that are sexually mature will deposit no ova, even 

 thongh the water is changed daily and they are well supplied with food. 



