134 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



doubtful whether such an experiment would succeed in central 

 Germany, for instance, or in the east of Europe. 



Everyone knows that in European countries animal life 

 exhibits a very marked periodicity. The greater number of our 

 birds quit us in the winter; many Mammals, Insects, Molluscs, 

 &c., hybemate in a condition resembling winter sleep ; others die 

 off altogether, as Sponges, Bryozoa, many Crustaceans, Insects, 

 &c., after laying eggs which survive the winter and are developed 

 in the spring. The creatures that live in streams and lakes, 

 nay, even on the sea-shore, breed only once, or at most twice, in 

 the year, some in the spring or summer, others late in the 

 autumn or at the beginning of winter, as the salmon. This 

 periodicity depends apparently on the direct effects of the severe 

 extremes of summer and winter temperature to which animals 

 are exposed when living in temperate continental climates. 

 This conclusion is easily proved by the following considerations. 



Every individual requires a certain duration of life to achieve 

 its individual development from the egg to sexual maturity 

 and full growth ; the length of time requisite for this is very 

 various, and, above all, bears no proportion to the size attained. 

 Animals grow at very different rates ; the minute Polystomum 

 which is parasitic in the bladder of the frog (fig 31) requires, 

 according to Zeller's latest investigations, about five years to 

 attain its full growth, while the Apus (fig. 33), a Crustacean of 

 much larger dimensions, and Branchipus reach their full size 

 in a few weeks of a warm summer. This length of time, which 

 we may generally designate as the period of individual growth, 

 is not alike even for all the individuals of the same species; 

 on the contrary, it depends on the co-operation of so many 

 different factors, that it must necessarily vary considerably. 

 Now, if from any cause the period of individual growth, say of 

 the salmon, became changed in consequence of the slower 

 development of the embryo in the egg or of the young larva, 

 most or all of the young salmon thus affected would die in our 

 climate, because the greater heat of spring is injurious to them 

 at that stage; or, on the other hand, an animal which is 

 usually hatched during the summer, so as to attain by the 

 autumn such a size as may enable it to resist the cold of the 



