198 A. S. PACKARD, JR., ON THE 



oviposition of the ova, I conceive to have been comparatively suddenly induced. It should 

 be remembered that at the present day we find great differences between the eggs and 

 mode of formation of the blastoderm in the eggs of the fresh water Gammarus and their 

 congeners previously alluded to. I conceive these differences to be due perhaps to sudden 

 changes of temperature in fresh water pools, to the difference in the density of fresh and 

 salt water, and the liability of fresh water pools to dry up, combined with less apparent 

 causes. We have indicated that Limulus develops from a primitive disc, like that of the 

 fresh water Astacus fluviatilis, whose mode of development is so unlike that of the marine 

 allied forms. In a more remarkable degree are sudden divergences shown in the species of 

 Eriphia, Crangon and Palsemon, whose development has been studied by Rathke (see p. 162). 

 In these cases the primitive band is more circumscribed, the germ starts differently and . 

 the metamorphosis is passed within the egg, instead of after hatching ; while, as is well known, 

 the other species of these genera are developed in the normal mode, the young being 

 hatched and passing through a series of complicated metamorphoses. Again, in the Gecar- 

 cinus we have one species which, according to Professor Westwood, undergoes no metamor- 

 phosis, the embryo being developed, as his figures indicate, as in Astacus and the other spe- 

 cies above noticed. A remarkable exception to the ordinary laws of oviposition occurs in 

 the salamanders. While most of our salamanders lay their eggs in the water, and the 

 young live in the same element, Mr. F. W. Putnam ^ has found the eggs and young of Ple- 

 thodon erythronohim under the moss and bark of decayed trees in the woods late in Au- 

 gust. A number of young had just hatched, as they retained their gills, which dropped 

 off about three days after hatching. Professor Baird (Tailed Batrachia of North America) 

 had also previously stated that the eggs of this species are "deposited in packets under 

 damp stones." 



Divergences from the normal mode of development of the group occurring at the present 

 day, are strong indications that the differences in the structure of the three groups of Crus- 

 taceans of the primordial zone were induced with comparative suddenness. Thus the accel- 

 erated and metabolous growth of the Nebaliadae point to a deeply seated law of growth per- 

 haps best expressed by the terms " acceleration and retardation of development," first pro- 

 posed by Prof Cope. And just as in the growth of the individual, the adult form is attained 

 by cell growth and the acceleration of growth in one part, and retardation in adjoining re- 

 gions, and sexual forms are determined by the unequal development of the primitive cells of 

 the generative organs in two different individuals, and just as the individual undergoes both 

 during and after embryonic life, a series of changes we call metamorphoses, which are simply 

 "expression points" in the life of the individual, — so we find a parallel series of similar causes 

 and effects in the evolution of the species, genera, orders, and higher groups of organized 

 beings. Prof Hyatt has shown (1866) that the development of the order is parallelled by 

 that of the individual in the Cephalopoda. It is therefore to the laws of ordinary growth, of 

 the production of the sexes, of metamorphosis and of parthenogenesis, that we must go for 

 an explanation of the laws of evolution. It should be borne in mind that though we see 

 individuals produced before our eyes and behold sexual differences and reproduction by 

 parthenogenesis, we do not yet know the causes that produce these effects. And so be- 

 cause we do not actually behold species and genera created by natural laws, it does not 



1 Proceedings Boston See. Nat. Hist., 1862, ix, p. 1 73. 



