PACKARD. — INHERITANCE OF ACQUIRED CHARACTERS. 340 



ably a secondary product, the result of adaptation to a free-swimming 

 mode of life; all its organs in a so much higher scale of perfection 

 than those of the generalized hydra-form ancestor having resulted from 

 the manifold stimuli of a changeable environment. The same may be 

 said of the Amelia, with its free Scyphistoma larval stage.* 



The alternation of generations in the Trematodes is apparently like- 

 wise the result of adaptation to a change of hosts bringing about homo- 

 chronous heredity. It is evident that alternation of generations is the 

 extreme of a series, beginning (1) with simple direct growth; (2) an 

 incomplete metamorphosis (the lower winged insects) ; (3) genuine 

 metamorphosis ; (4) the hypermetamorphosis of Meloidre, Rhipi- 

 phoridae, and Stylopidae ; the oth and last term being the cases of 

 alternation of generations. It seems difficult to account for these 

 sets of individuals or generations, unless we resort to the principle of 

 inheritance of characters acquired during the lifetime of the ancestral 

 forms, which gave rise to these interrupted or alternate series of forms. 



On the other hand Weismann in order to account for alternation of 

 generations carries us out of the sphere of observation and induction 

 into speculative regions, and assumes that >k two kinds of germ plasm 

 exist \n those species in xohich alternation of generations occurs, both 

 of which are present in the egg cell as well as in the bud, though only 

 one of them is active at a time and controls ontogeny, while the other 

 remains inactive. The alternating activity of these two germ plasms 

 causes the alternations of generations." | Are not over-nutrition and 

 changes in station and habits the more appreciable and potent causes ? 



The complicated metamorphosis of the Crustacea is the result of the 

 adaptation to variations in the environment. It is not improbable that 

 the Nauplius of the primitive Crustacea, the Branchiopoda (including 

 the Phyllopoda), was a secondary and not a primary form. The proof 

 would seem to be the non-existence of any adult Arthropod with such 

 a form and structure as that of the Nauplius. The earliest Crustacean 

 was probably a naked, shelless Cladoceran or Phyllopod-like form, 

 with a few or many definite segments, bearing unjointed or imperfectly 

 jointed lamellate swimming legs, derived from the flattened parapodia 



* In his suggestive book entitled " A Theory of Development and Heredity " 

 (1803), Prof. H. B. Orr accounts for alternation of generations by secondary 

 changes of the environment which favored the hydroid stage and the perfect 

 medusa stage, and at the same time tended to eliminate the intermediate stages. 

 In other cases the secondary changes of environment destroye 1 the hydroid 

 stage, as we find Medusae without any hydroid stage, (p. 22-3 ) 



t The Germ Plasm, p. 457. 



