502 LIFE HISTORY STUDIES OP ANIMALS. 



processes and the course of development are as liable to vary for motives 

 of expediency as the form of a leg or fin. The supposed constancy (the 

 necessary constancy according to some naturalists) of the embryonic 

 stages throughout large groups would not be hard to break down, if it 

 were to be again asserted. Probably the doctrine is now totally aban- 

 doned ; it belongs to that phase of zoological knowledge in which Meckel 

 could declare that every higher animal passes in the course of its devel- 

 ment through a series of stages which are typified by adult animals of 

 lower grade, and when an extreme partisan, far inferior to Meckel both 

 in experience and caution, could affirm that the human embryo omits 

 no single lower stage. 



The tadpole larva, which is common in lower vertebrates and their 

 allies, shows the influence of adaptation as strongly as any larva that 

 we know. We may describe the tadpole as a long-tailed chordate, 

 which breathes by gills and has a suctorial mouth disk, at least during 

 some part of its existence. It is a cheap form of larva, when reduced 

 to its lowest terms, requiring neither hard skeleton, nor limbs, nor neck, 

 yet it can move fast in water by means of its sculling tail. Such a 

 tadpole appears in many life-histories, and plays many parts. The tad- 

 pole is the characteristic tunicate larva, and in this group commonly 

 ends by losing its tail, and becoming fixed for life. But Salpa, which 

 is motile when adult, has lost its tadpole. Appendicularia has lost the 

 normal adult stage if it ever had one, and its tadpole becomes sexually 

 mature. The same thing seems to have happened to many amphibia, 

 whose tadpoles acquire legs, become sexually mature, and constitute 

 the normal adult stage. The lamprey, as Balfour and others have 

 recognized, is another kind of sexually mature tadj^ole. Thus the tad- 

 pole may act as larva to a sea squirt, fish {Acipenser, Lepidosteus, 

 Amia), or frog; it may also constitute the only remaining stage in the 

 free life history. 



The lower and smaller animals seem to show beyond others the prev- 

 alence of adaptive features. They offer visible contrivances of infinite 

 variety, while they are remarkable for the readiness with which new 

 stages are assumed or old ones dropped, and for their protean changes 

 of forms, which are so bewildering that many worms, for instance, can 

 not as yet be placed at all, while many larvai 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, alter- 

 nation of generations, and spore formation (as in Gregarina) are ulti- 

 mately 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 oft", others remain, that no enemy can 



