38 GENESIS OF THE ARIETIDA. 
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 
correlation 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,”’* 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 
Cephalopods.”’ ? 
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. 
If 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.’ 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. IL. pp. 18, 22, 320. 
2 Mem. Bost. Soc. Nat. Hist., I. p. 195 et seq., and Proc. of same, X., 1866, p. 302. 
8 See Genera of Fossil Cephalopoda, p. 261, and Fossil Cephal. Mus. Comp. Zodl., p. 339. 
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