90 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



torpid under a degree of warmth which will be sufficient to arouse 

 another of the same kind, accustomed to a somewhat colder climate ; 

 because the stimulus is relatively greater to the latter. 



182. It was observed by Mr. Darwin, that at Bahia Blanca in South 

 America, the first appearance of activity in animal and vegetable life, a 

 few days before the vernal equinox, presented itself under a mean tem- 

 perature of 58, the range of the thermometer in the middle of the day 

 being between 60 and 70. The plains were ornamented by the flowers 

 of a pink wood-sorrel, wild peas, evening primroses, and geraniums ; 

 the birds began to lay their eggs ; numerous beetles were crawling about ; 

 and lizards, the constant inhabitants of a sandy soil, were darting about 

 in every direction. Yet a few days peviously, it seemed as if nature 

 had scarcely granted a living creature to this dry and arid country ; 

 and it was only by digging in the ground that their existence had been 

 discovered, several insects, large spiders, and lizards, having been found 

 in a half-torpid state. Now at Monte Video, four degrees nearer the 

 Equator, the mean temperature had been above 58 for some time pre- 

 viously, and the thermometer rose occasionally during the middle of the 

 day to 69 or 70; yet with this elevated temperature, almost equivalent 

 to the full summer heat of our own country, almost every beetle, several 

 genera of spiders, snails, and land-shells, toads and lizards, were still 

 lying torpid beneath stones. We have seen that at Bahia Blanca, whose 

 climate is but a little colder, this same temperature, with a rather less 

 extreme heat, was sufficient to awake all orders of animated beings ; 

 showing how nicely the required degree of stimulus is adapted to the 

 general climate of the place, and how little it depends on absolute tem- 

 perature. 



133. We may learn much from the Geographical distribution of the 

 different species of cold-blooded animals, in regard to the influence of 

 temperature on Animal life. No general inferences of this kind can 

 be found upon the distribution of warm-blooded animals ; since their 

 own heat-evolving powers make them in great degree independent of 

 external warmth. And it is probably from the distribution of the 

 marine tribes, whose extension is less influenced by local peculiarities, 

 that the most satisfactory deductions are to be drawn. In regard to 

 the class of Crustacea, which is the one that has been most fully inves- 

 tigated in this respect, the following principles have been pointed out 

 by M. Milne Edwards ; and they are probably more or less applicable 

 to most others. 



I. The varieties of form and organization manifest themselves more, 

 in proportion as we pass from the Polar Seas towards the Equator. 



II. The differences of form and organization are not only more nume- 

 rous and more characteristic in the warm than in the cold regions of 

 the globe ; they are also more important. 



in. Not only are those Crustacea, which are most elevated in the 

 scale, deficient in the Polar regions ; but their relative number increases 

 rapidly as we pass from the Pole towards the Equator. 



iv. When we compare together the Crustacea of different parts of 

 the world, we observe that the average size of these animals is con- 



