38 GENESIS OF THE ARIETID.E. 



the latter of the more specialized ephebolic characteristics which arose in the 

 group during its acme of evolution in time. 



We now propose to take one step more, and try to show that this tripartite 

 con-elation in the development of the individual and in the evolution of the 

 type correlates with a similar cycle in the modes of development of the indi- 

 vidual, which we have classified as the direct radical, or anaplastic mode, the 

 complex, or metaplastic mode, and the direct geratologous, or cataplastic mode. 

 We are thus carrying to definite conclusions and confirming by application the 

 laws announced by Haeckel in his '-Morphologie der Organismen," 1 and by the 

 author independently with regard to the ontogeny of the Cephalopoda, in his 

 essay on the " Parallelism between the Individual and Order in Tetrabranchiate 

 Cephalopoda." 2 



Haeckel was a strong advocate of the general efficacy of natural selection 

 as a motive force of far greater importance in the evolution of types than has 

 been granted in this and other publications by the author, and did not give as 

 much weight to the correlations between the ontogenetic and phylogenetic cycles. 

 II these have the correlations here claimed, then we can see that a theory like 

 that of natural selection, which does not recognize the action of some law which 

 has affected both the individual and the type in the same way, cannot be recon- 

 ciled with the observed facts in the evolution of forms. A theory of evolution 

 must necessarily, while admitting the origin of new characters by external causes, 

 also recognize the limitations due to the force of heredity in conserving the type. 

 It must admit fully the plasticity of organisms, and the power of external condi- 

 tions to effect fundamental changes in structure by means of internal reactions, — 

 that is, through the action of either conscious or unconscious effort, — but not 

 deny to heredity its fullest effect in the tendency of like to produce like. It 

 should emphatically deny that heredity tends to produce like- with variations, or 

 that there is any such thing as a tendency to variation which is inherent and 

 not produced by external forces. It must recognize not only the three physio- 

 logical phases of epacme, acme, and paracme, but that all the phenomena of 

 evolution accord with this cycle. It should show that not only the general 

 physiological phenomena, but also the relative strength of the individual, as 

 testified by the slight effect of old age upon the shell in the oldest and 

 simplest types, and the more rapid evolution of paleozoic as compared with 

 mesozoic types, and of geratologic types at the termini of each special group of 

 forms in time, are strictly in accord, and testify to the existence of a common 

 law governing and producing cycles. It must also recognize that there is a 

 growing stability in types, and less important structural variations at the acme of 

 a group than during its epacme or paracme ; and that its habitat was freer 

 at first than it was subsequently during its acme. 8 The struggle for existence 

 between species, if there were any at first, (which we do not believe,) must have 

 been very slight compared with what it became during the crowded acme, and 



1 Vol. II. pp. IS. 22, 320. 



Mem. Bost. Soc. Nat. Hist., 1. p. 195 ei seq., and Pioc. of same, X., 1866, p. 302. 

 8 See Genera of Fossil Cephalopoda, p. 261, and Fossil Cephal. Mus. Comp. Zool., p. 339. 



