LIFE HISTORY STUDIES OF ANIMALS. 503 



possibly devour all the medusae liberated from one colony, or all the 

 planulne liberated from one medusa. Low organization gives very 

 special facilities for extreme division. There are animals and plants 

 which multiply greatly as a consequence of being torn to pieces or 

 chopped small (chigoe, some fungi, etc.). 



Small animals are usually short-liyed. Many complete their life- 

 history in a few weeks. Those which last for so long as a year are 

 ofteu driven, like annual plants, to adapt every detail of their existence 

 to the changing seasons. The naturalist who explores the surface 

 waters of the sea with a tow net soon learns that the time of year 

 determines the presence or absence of particular larva?. It is probably 

 as important to an Aurelia as to a butterfly that it should tide over the 

 storms of Avinter by means of a sedentary and well-protected stage. 

 Auy one who keeps scyphistoma in an aquarium will remark how small 

 it is, how it creeps into crevices or the hollows of dead shells,- But 

 when the depth of winter is j)ast it pushes out its strobila, which in 

 spring liberates ephyrsB. These rapidly enlarge, and by August have 

 grown from microscopic disks to jelly-fishes a foot across. 



The intelligence of many small animals is very low. They go on 

 doing the thing that they have been used to do, the thing that has 

 commended itself to the experience of many generations. They are 

 governed by routine, by that inherited and unconscious power of 

 response to external stimulus which we call instinct. But there are 

 some notable exceptions. Of all small animals, insects seem to show 

 the greatest flexibility of intelligence. 



There is one large group of animals w^hich is in striking contrast to 

 nearly all the rest. Vertebrates, and especially the higher vertebrates, 

 are usually big and strong. They rely upon skill, courage, or some 

 other product of high organizations rather than upon numbers and 

 fertility. Vertebrates swallow many other animals, together with their 

 living parasites, but are rarely swallowed alive or fresh by invertebrates. 

 This fact of nature has led to many consequences, among others to this, 

 that many parasites which pass their earlier stages in the bodies of 

 invertebrates only attain sexual maturity in a vertebrate host. The 

 complexity of the structure of a vertebrate iirecludes the possibility of 

 multiplication by breaking-up or budding, and they multiply only by 

 egg-laying or strictly analogous processes. The higher vertebrates live 

 so long that the accidents of a particular year or a particular season 

 are not of vital importance. Hence seasoual transformation is almost 

 unknown; the quadruped or bird may choose the warm months for 

 rearing the family, or celebrate the pairing season by getting a new suit 

 of feathers, or grow a thicker coat against the cold of winter, but that 

 is all, Ko vertebrates perish regularly at the approach of winter, 

 leaving only batches of eggs to renew the species in spring, nor is their 

 structure profoundly modified by the events of the calendar (the frog 

 is a partial exception). One minor cause of transformation, which 



