LIFE HISTORY STUDIES OF ANIMALS. 501 



migratory stage, such a stage will be added. In certain decapod Crus- 

 tacea {Penceus, Leucifer) a nauplius and as many as five other stages 

 may interveue before the final or adult stage. Some of these larval 

 stages are common to a great many Crustacea, but none, as we now 

 think, belong to the original phylogeny. If a resting or a winged stage 

 is wanted, it is supplied just as easily — witness the holometabolic insects. 

 Here again, so far as we know, there is nothing absolutely new.' The 

 stages which seem new are merely exaggerations for special purposes 

 of sections of the life history, which were originally marked out by 

 nothing more important than a change of skin and a swelling out of the 

 body. Let us not suppose for a moment that it is a law of insect devel- 

 opment that there should be larva, pupa, and imago, or that it is a law 

 of crustacean development that there should be six distinct stages 

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

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

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

 the embryonic molts of some centipedes and spiders, Liniuh(s, many 

 Crustacea, and Podura. The parthenogenetic reproduction of some 

 immnture 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 Lueernaria, 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, and JEpicauta are well-known examples. 

 In some ephemeridaj the molts, which are potential stages, become 

 very numerous, but as a curious exception to a very general rule, the 

 last molt 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 adapta- 

 tion 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 poecilogony. A number of very difierent ani- 

 mals^ 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, etc., can not 

 escape notice, and Giard connects with all these the paedogenesis 

 of Miastor and Ghironomus, and many cases of heterogony. For 

 our immediate purpose it is sufficient to remark that the reproductive 



iNirgends ist Neubildung; sonder nur Umbildung. — Baer. 

 2 C.R. 1891, 1892. 



^E. g. Crustacea (Palwmonetes, Alpheus), insects {Musca corvina, some lepidoptera 

 and diptera), an ophiurid (OpMothrix), a compound ascidian {Le})loclinus ) , etc. 



