4o8 



NATURE 



[August 26, 1897 



Daphnia, &c,, cannot escape notice, and Giard connects with 

 all these the pEcdogenesis of Miastor and Chironomus, and many 

 cases of heterogony. For our immediate purpose it is sufficient 

 to remark that the reproductive processes and the course of de- 

 velopment 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 

 abandoned ; it belongs to that phase of zoological knowledge in 

 which Meckel could declare that every higher animal passes in 

 the course of its development 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-disc, 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 tadpole 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 tad- 

 pole becomes sexually mature. The same thing seems to have 

 happened to many Amphibia, whose tadpoles acquire legs, be- 

 come sexually mature, and constitute the normal adult stage. 

 The Lamprey, as Balfour and others have recognised, is another 

 kind of sexually mature tadpole. Thus the tadpole 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 prevalence of adaptive features. They offer visible con- 

 trivances 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, cannot as yet be placed at all, 

 while many larvae 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 

 medusoe liberated from one colony, or all the planulre liberated 

 from one medusa. Low organisation gives very special facili- 

 ties 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 tim^e of year determines the presence or absence of 

 particular larvae. 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 ephyrce. 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 genera- 

 tions. They are governed by routine, by that inherited and 

 unconscious power of response to external stimulus, which we 



NO. 1452, VOL. 56] 



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 con- 

 trast to nearly all the rest. Vertebrates, and especially the 

 higher Vertebrates, are usually big and strong. They rely 

 upon skill, courage, orsome*Other product of high organisations, 

 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 pre- 

 cludes the possibility of multiplication by breakihg-up or bud- 

 ding, and they multiply only by egg-laying or strictly analogous 

 processes. The higher Vertebrates live so long that the acci- 

 dents of a particular year or a particular season are not of vital 

 importance. Hence seasonal 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. 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 aff"ects the life-history 

 of many polyps, worms and insects, is thus removed. Verte- 

 brates often take care of their young, and the higher Verte- 

 brates bring forth few at a time. For this reason among others 

 they rarely afibrd examples of free larvre. Such Vertebrate 

 larv£e 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 Am- 

 mocoetes, 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 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 de- 

 velopment. Yet the Sauropsida and the Mammalia are posi- 

 tively known to go back to the earliest secondary and late 

 palaeozoic 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 Sauropsida and Mammalia, in com- 

 parison with other animals, are particularly 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 

 immature stages, and some other slight hints, that their life is 

 a hard one. 



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

 piscine structure which are no longer useful has been ascribed 

 to a principle called Recapitulation, and Haeckel lays it down 

 as a fundamental biogenetical 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 Recapitu- 

 lationist demands — for instance, that certain facts in the de- 

 velopment 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 statement 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 

 and simple — a thing which is common in Physics, but very 



