120 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



promptly fatal. Animals of the North Sea, when exposed to a 

 temperature of 30 under the direct rays of the sun, die, it 

 would seem, at once, and invariably, while this appears to be 

 the optimum of warmth for the Branchipus stagnalis, living in 

 our pools. Animals living within the tropics are generally 

 exposed to a much higher and more equable temperature than 

 our northern or boreal forms, and few species can long survive 

 a transfer from one region to the other; 1 shall presently return 

 to this subject. It certainly appears surprising, though it is an 

 indisputable fact, that certain animals and even plants are capable 

 of enduring a higher temperature 42 than is generally fatal, as it 

 would seem, to protoplasm, which is the fundamental element of 

 all organic life. It is known that animal protoplasm iisually 

 coagulates and dies at 40 centigrade, and always at 50, and this 

 agrees with what is known of vegetable protoplasm. Hence 

 we should feel inclined to conclude that animals could not 

 exist in spots as, for instance, in many hot springs where 

 the temperature exceeds the maximum limit in which proto- 

 plasm can live. But this assumption is in contradiction to 

 ascertained facts. It is superfluous to give these in detail; it 

 will suffice to state that animals of tolerably high grade, as 

 Crustacea, the larvae of insects, &c., live in springs having a 

 temperature of 50-60 or even more. In view of the numerous 

 data on this subject, and the trustworthiness of most of the 

 observers, we are not justified in doubting the facts, although 

 the physiological puzzle which they offer cannot at present be 

 solved indeed, no attempt has ever been made to solve it. 

 We might have recourse to the supposition that the capability 

 of many animals for living and multiplying in such heat must 

 result from a capability in their protoplasm for resisting the 

 injurious effects of a temperature of 50 centigrade and more. 

 But this would prove far greater powers of adaptation in animal 

 protoplasm than have yet been considered possible. 



It might perhaps seem plausible to many readers to assume 

 that immxmity from the injurious effects of too great heat 

 bore some analogy to immunity from injury from too great 

 cold. But such a parallel is not admissible from a closer and 

 more exact point of view. Every fall of temperature below 



