TRANSACTIONS OF SECTION D. 681 



many Worms, for instance, cannot as yet be placed at all, while many larvre give 

 no clue to their parentage. These lower and smaller animals show beyond others 

 a tendency to multiply rapidly, and to break away from one another in an early 

 stage. The tendency is so strong in the microscopic Protozoa that it enters into 

 the definition of the group. Fission, budding, alternation of generations, and 

 spore-formation (as in Gregarina) are ultimately due to the same tendency. 



Weak animals are almost inevitably driven to scatter, and to make up by their 

 insignificance, their invisibility, and their powers of evasion for the lack of power 

 to resist. It is a great thing to a Hydrozoan colony that if one polyp is bitten off", 

 others remain, that no enemy can possibly devour all the medusae liberated from 

 one colony, or all the planulae liberated from one medusa. Low organisation 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, &c.) 



Small animals are usually short-lived. Many complete their life-history in a 

 few weeks. Those which last for so long as a year are often 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 larvse. It 

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

 storms of winter by means of a sedentary and well-protected stage. Any 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 past, 

 it pushes out its strobila, which in spring liberates ephyrse. These rapidly enlarge, 

 and by August have grown from microscopic discs 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 which 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 organisa- 

 tions, 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 precludes 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 par- 

 ticular j'ear or a particular season are not of vital importance. Hence seasonal 

 transformation is almost unknown ; the quadruped or bird rtiay 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. 

 No 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 affects the life history f many polyps, worms and insects, 

 is thus removed. Vertebrates often take care of their young, and the higher 

 Vertebrates bring forth few at a time. For this reason among others they rarely 

 afi'ord examples of free larvse. Such Vertebrate larvse as we do find, conform to 

 the Vertebrate type. It is often impossible to predict what adult will develop 

 from an Invertebrate larva, but no one could hesitate to rank an Ammocoetes, a 

 Leptocephalus, or a tadpole among the Vertebrates. 



It accords with this strength and mastery that Vertebrates, and especially the 

 higher Vertebrates, should be more stable, more conservative, less experimental 

 than other animals. They retain ancient structures long after they have ceased to 



