TAXONOMY, ANATOMY, EMBRYOLOGY 



lost, and the parasite becomes sac-like, with branches invading the tissues 

 of the host like a tumor and absorbing nutriment. The only organ system 

 which is well developed is the reproductive system. This parasite re- 

 sembles no other organism, and so it defied classification luitil the larvae 

 were studied. The larvae showed clearly that the parasite could be noth- 

 ing else but a much degenerated barnacle. The case of the enteroxenid 

 snails which parasitize echinoderms is quite similar. They are worm-like 

 externally. Internally, they have only a much reduced gut and an her- 

 maphroditic reproductive system. Yet they produce typical snail larvae. 



Another interesting case which was originally studied by Darwin is that 

 of the loggerhead duck, or steamer duck, Tachyeres cinereus. This bird is 

 widely distributed in southern South America, the Straits of Magellan, 

 and the Falkland Islands. The young ducks are fairly good fliers, but the 

 growth of the wings does not keep pace with the growth of the body, and 

 the adults are completely flightless. 



Examples Among Plants. The embryology of plants is generally sim- 

 pler than that of animals, and hence the recapitulation principle is not so 

 abundantly exemplified. Yet there are good examples available in the 

 plant kingdom. The Acacia tree, for example, has highly compound leaves, 

 yet its seedlings have simple leaves like its ancestors. Adult cactus plants 

 have no typical leaves at all, although these may be represented by the 

 needles. But the seedlings have readily recognizable leaves. To give one 

 more example utilizing a leaf character, the live oaks of southern United 

 States, which retain their foliage the year around, are considered to be 

 more primitive than the northern species which are deciduous. However, 

 the saplings of northern oaks commonly retain their leaves during the 

 winter, thus recapitulating what appears to have been an ancestral char- 

 acter. 



Jeffrey has found a very interesting and possibly comparable phenome- 

 non in conifers. If conifers are injured and the wound allowed to heal, the 

 new growth may differ histologically from normal tissue. In such in- 

 stances, the new tissue shows a type of structure which is well known 

 from fossil conifers of the Mesozoic era. 



Difficulties of the Biogenetic Law. Because of numerous examples like 

 those described, many biologists of Haeckel's time thought that embryol- 

 ogy, when sufficiently known, would be a golden key to problems of 

 phylogeny. Yet there was much unsound biology associated with the Bio- 

 genetic Law, and few aspects of evolutionary science have been so heavily 

 attacked in recent years. The reasons are simple enough. The recapitula- 

 tion theory assumes that embryos need only repeat the past, condensing 

 some stages, eliminating others, without adaptation to the embryonic 

 mode of life. Actually, the embryo must cope with a hostile environment, 

 even as do adults. All pelagic larvae are subject to heavy predation. The 

 first chapter in embryology texts frequently deals with differences in 

 cleavage patterns which are correlated with amounts of yolk in the eggs, 

 an adaptive trait of fundamental biological importance. Fetal membranes 

 of amniotes are an obvious series of embryonic adaptations. Needham has 

 described an interesting series of adaptations of eggs to fresh water, to 



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