234 



EVOLUTION AND ANIMAL LWE 



developmental stages, so that at that time it looks like and 

 really is like the mature stage of some tailed crustacean like 

 a crayfish. A barnacle, which looks little like a crayfish or 

 prawn in its mature stage, is hardly to be distinguished in its 

 immature life from a young shrimp or prawn. Sacculina, which 

 is a still more degenerate crustacean, is only a sort of feeding 

 sac with rootletlike processes projecting into the body of the 

 host crab on which it lives as a parasite, but the young free- 

 swimming Sacculina is essentially 

 like a barnacle, crayfish, or crab 

 in its young stage. 



However, it is obvious that 

 this recapitulation or repetition 

 of ancestral stages is never per- 

 fect, and it is often so obscured 

 and modified by interpolated 

 adaptive stages and characters 

 that but little of an animal's 

 ancestry can be learned from a 

 scrutiny of its development. 

 The fascinating biogenetic law 

 of Miiller and Haeckel summed 

 up in the phrase, "ontogeny is 

 a recapitulation of phylogeny,'' 

 must not be too heavily leaned 

 on as a support for any specula- 

 tions as to the phyletic affinities of any species or group of 

 species of organisms. " Embryology is an ancient manuscript 

 with many of the sheets lost, others displaced, and with 

 spurious passages interpolated by a later hand.'' 



While a young robin when it hatches from the egg or a 

 young kitten at birth resembles its parents, a young starfish 

 or a young crab or a young butterfly when hatched does not 

 at all resemble its parents. And while the young robin after 

 hatching becomes a fully grown robin simply by growing larger 

 and undergoing comparatively slight developmental changes, 

 the young starfish or young butterfly not only grows larger, 

 but undergoes some very striking developmental changes; the 

 body changes very much in appearance. Marked changes in 

 the body of an animal during postembryonic or larval de- 

 velopment constitute what is called metamorphic development, 



Fig. 137. — Metamorphosis of a bar- 

 nacle, Lepas: a. Larva; b, adult. 



