680 EEPORT— 1897. 



distinct stages between the egg and the adult. Any of these stages may be 

 dropped, if it proves useless — either totally suppressed, or telescoped, so to speak, 

 into the embryonic development. Lost stages are indicated by the embryonic 

 moults of some centipedes and spiders, Limulus, many Crustacea, and Podura. 

 The parthenogenetic reproduction of some immature insects, such as Miastor, 

 shows a tendency to suppress later stages. Perhaps the wingless Thysanura are 

 additional examples, but here, as in the case of Hydra and Lucernaria, we do not 

 certainly know whether they are primitive or reduced. It seems to be easy to add 

 new stages, when circumstances (and especially parasitism) call for them. Meloe, 

 Sitaris, andEpicauta are well-known examples. In some Ephemeiidse the moults, 

 which are potential stages, become very numerous, but as a curious exception to a 

 very general rule, the last moult of all, which is usually so important, may be 

 practically suppressed. The fly of an Ephemera may mate, lay eggs, and die, 

 while still enveloped in its last larval skin. 



Among the many cases of what one is inclined to call rapid adaptation to 

 circumstances (the chief indications of rapidity being the very partial and isolated 

 occurrence of remarkable adaptive characters) are those which Giard ' has collected 

 and compared, and which he refers to a process called by him Pcecilogouy. A 

 number of very different animals - produce according to habitat, or season, or some 

 other condition closely related to nutrition, eggs of more than one sort, which 

 differ in the quantity of nourishment which they contain and in the degree of 

 transformation which the issuing larva is destined to undergo. The analogy with 

 the summer and winter eggs of Daphnia, &c. cannot escape notice, and Giard 

 connects with all these the psedogeuesis 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 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 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 tadpole becomes sexually mature. The same thing seems to have happened to 

 many Amphibia, whose tadpoles acquire legs, become sexually mature, and consti- 

 tute 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 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 



' C. R. 1891, 1892. 



- E.g. Crustacea (Palsemonetes, Alpheus), Insects (Musca corvina, some Lepidoptera 

 and Diptera), an Ophiurid (Ophiothrix), a Compound Ascidian (Leptoclinus), &c. 



