422 TEMPERATURE AND LIFE. 



the (lay when a .seed germinates to the moment when the plant reaches 

 its maturit}' an arerage is taken of the temperature for eacii cycle of 24 

 hours. Afterwards an average is made of these averaf?es for all the 

 period which lias i)assed between the two moments mentioned above and 

 this average is to be multiplied by the number of days which have passed. 

 Suppose this action of the plant has taken 00 days, and that the aver- 

 age of averages is 17, then you obtain the figure 1530, which represents 

 the degrees of heat furnished in 90 days, — a day being taken as a nnit of 

 time. A very interesting fact is, that, if the same observations are made 

 with the same species of plant under dittereut thermic conditions, or in 

 a dittereut climate, the same figure is obtained, although the number 

 of days necessary to the develoi)ment may vary from simple to treble, 

 according to the climate. The study of vegetable physiology is rich in 

 interesting facts from the standpoint which is now occui)ied. In this 

 way difterent seeds are very diff"erently influenced by cold. One does 

 not germinate below 15°, while others germinate at 4°, and still others 

 at zero. One plant developes best at a temperature which is fatal to 

 another. 



In the animal kingdom analogous facts have been observed in a very 

 exact manner. A little fresh-water mollusk (lymnee) furnishes Carl 

 Semi)er, the learned zoologist of Wiirzburg, with very interesting 

 facts in this connection. Below 12° this animal, although leading an 

 active life and taking its food regularly, underwent no growth, though 

 it was able to reproduce, its eggs developing perfectly. From 12° to 

 250 (which is its most favorable temperature) its assimilation was per- 

 fect, and the animal grew and developed. Semper remarks that these 

 mollusks, subjected permanently to a temperature of 10° or 12°, remain 

 small and cease to develop. They produce a dwarfed breed, which in 

 their turn reproduce normally, remaining, however, smaller than the 

 other lymndes. On the other hand, an unnaturally large species can be 

 produced by maintaining the mollusks by artificial means at the highest 

 point of temperature. There is still another fact which accords with that 

 of which we have just spoken. A well-known naturalist, Mtebius, has 

 discovered that the same species of marine mollusks common to the 

 Baltic and to the coast of Greenland differ greatly in size. At the 

 Baltic they are small and have a thin sliell, while on the coast of Green- 

 laud they are much larger in size and are provided with a thick shell. 

 This is explained by the fact that in the Baltic the variations of tem- 

 perature are more frequent and the cold is more intense than in Green- 

 land, in consequence of which the development of the mollusk is more 

 difQcult and intermittent. 



Temperatures lower than this most favorable point have a marked 

 eflect upon animals and plants, which shows itself in the latter by a 

 retardation of development which at the same time becomes less com- 

 plete. On the contrary, temi)eratures not fatal, but relatively high in 

 regard to their natural condition, favor their growth, which becomes 

 proportionately rapid and complet^j. It is thus with the eggs of certain 



