126 Dynamic Theory. 



creatures having different powers of resistance. At 5 their pulsations 

 still continued, but at 2 or 3 all movement ceased. These animals 

 have two kinds of Cilia, those which produce a rotary motion and are 

 involuntary, and those which move backward and forward and are vol- 

 untary or subject to the will. The involuntary movements continue up 

 to a temperature of 42 to 45 C. , but the voluntary movements cease at 

 40 and become wild and purposeless at 35. From the torpor pro- 

 duced by 5 C. these specks of vitality will recover if it be not too pro- 

 longed. The pond snail Lymnsea Stagnalis will not die in a tempera- 

 ture close to freezing, but will not assimilate food and grow below a 

 temperature of 12, the normal temperature being 25 C. If raised in a 

 low temperature they are dwarfed in size but not injured in their repro- 

 ductive powers. In fact, the best temperature for reproduction is much 

 lower than for growth, and the spermatozoa and ova do not seem to be 

 formed till the growth of the other tissues is checked, which occurs in 

 the winter and early spring, when the reproduction takes place. Cold 

 as a rule has a tendency to dwarf all organic growth, both vegetable and 

 animal, the degree of temperature to be called cold, however, being very 

 different in different organisms. But the checking of growth affects 

 some tissues sooner than others. The reproductive powers for obvious 

 reasons are usually among the last to suffer from a decline of vitality. 

 Reproduction is the great conservative function, appearing, as the cul- 

 mination of all the rest, and when they receive their first check or halt 

 in development. The check in general development caused by the cold 

 of winter is followed in a great many cases by the formation of repro- 

 ductive germs. The eggs of frogs and snails are formed in the ovaries 

 in the winter at a time when the animals eat little or nothing and while 

 their temperature is too low to assimilate food. They are not spawned, 

 however, till the temperature begins to rise in the spring. But even 

 the immature young or larvae of many animals are able to reproduce if 

 their growth be checked. It is to be observed that the exhaustive ef- 

 fects of excessive heat are similar to those of excessive cold. Growth 

 and development may be stinted and checked by one equally with the 

 other. Accordingly we find that half grown land snails will reproduce 

 when their growth is checked by cold, and they will also reproduce 

 when their growth is checked at the age of six months by intense heat. 

 Then after the summer is over, on the approach of cold weather, the 

 animal being nearly or quite grown, another deposit of eggs takes place. 

 This reproduction by stinted youngsters is by no means rare ; it happens 

 to the larvae of worms, Ascidians, Mollusks and even vertebrates, as in 

 the case of the Axolotl. 



Among crustaceans, insects, rotatoria and others there are many 

 species that lay two sets of eggs, one in summer and the other in 



