504 LIFE HISTORY STUDIES OF ANIMALS. 



affects the life history of 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 afford examples of free larvae. Such vertebrate larvae as 

 we do find conform to the vertebrate type. It is often impossible to 

 predict what adult will develop from an invertebrate larvae, 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 conserv- 

 ative, less experimental than other animals. They retain ancient 

 structures long after they have ceased to be useful. The gill clefts, 

 gill arches, and branchial circulation are good examples. Though not 

 functional in sauropsida and mammalia, they never fail to appear in 

 the course of the development. Yet the sauropsida and the mammalia 

 are positively known to go back to the earliest secondary and late pale- 

 ozoic times. Ever since the beginning of the secondary period at least, 

 every reptile, bird, and mammal has continued to pass through a stage 

 which seems obviously piscine, and of which no plausible explanation 

 has ever been offered, except that remote progenitors of these animals 

 were fishes. Could not natural selection, one is tempted to ask, have 

 straightened the course of development during lapses of time so vast 

 and have found out less roundabout ways of shaping the tongue bone 

 and the ossicles of the ear? Either it costs nothing at all to pursue 

 the old route, or it costs nothing which a higher vertebrate will ever 

 miss. The second alternative seems to me the more likely. The sau- 

 ropsida and mammalia, in comparison with other animals, are particu- 

 larly well off', and like wealthy housekeepers they do not care what 

 becomes of the scraps. It is, I fancy, different with many fishes, which 

 show by their numerous eggs, the occasional presence of peculiar imma- 

 ture stages, and some other slight hints, that their life is a hard one. 



The presence in the developing reptile, bird, or mammal of piscine 

 structures which are no longer useful has been ascribed to a principle 

 called recapitulation, and Haeckel lays it down as a fundamental bio- 

 genetical law that the development of the individual is an abbreviated 

 recapitulation of the development of the race. If I had time to discuss 

 the recapitulation theory, I should begin by granting much that the 

 recapitulationist demands — for instance, that certain facts in the devel- 

 opment of animals have an historical significance, and can not be 

 explained by mere adaptation to present circumstances; further, that 

 adaptations tend to be inherited at corresponding phases both in the 

 ontogeny and the phylogeny. I am on my guard when he talks of laws, 

 for the term is misleading, and ascribes to what is a mere general state- 

 ment of observed facts the force of a command. The so-called laws of 

 nature (a phrase to be avoided) may indeed enable us to predict what 

 will happen in a new case, but only when the conditions are uniform 



