ORIGIN OF DIFFERENTIALS. ays) 
We do not intend to dispute entirely the action of natural selection and 
the influence of the struggle for existence, but simply to deny the applicability 
of the law to the more important modifications and series of modifications which 
have occurred in the history of animals, taking the fossil Cephalopoda as a type. 
We have in former papers conceded the preservation of differentials to the law 
of natural selection, rather on account of the apparent logical necessity of thus 
accounting for the invariability of minute differences, like the ventral position of 
the siphon, the siphonal collar, the short funnel, the convexity of the septa and 
ventral lobe of Ammonoids, and the divergence of the type from the common 
stock of Nautiloids which these characteristics indicated, rather than from any 
firm conviction derived from analytical study. 
We think now that the changes in the surroundings acted upon the plastic 
organism, inducing it to make efforts to accommodate itself to new conditions. 
Effort, being a reaction from within upon a common organization, necessarily 
produced similar series of modifications whenever the surroundings were not 
changed so completely as to lead the phylum away from the original type on 
lines of extreme divergence. Thus parallelisms occurred between the differen- 
tials of the Nautiloidea and Ammonoidea, they were less apparent in Belemnoids, 
which are more remote in habitat, and may be said to have been almost wholly 
absent in Sepioids, which are still more remote in habitat, as pointed out above. 
It passes without saying that the differentials are in many cases new modifica- 
tions; and, if our position is true, they are adaptive characters, correlative in the 
Nautiloids with their mixed habitat as swimmers and crawlers, in the Ammonoids 
with their habitat as reptant forms, in the Belemnoids with their intermediate 
habitat as leapers or bottom swimmers, and in the Sepioids with their habitat as 
surface swimmers. The simple lobes and saddles, keelless abdomens, and ab- 
dominal sinuses in the shells of the Nautiloids, the dendritic or deeper lobes 
and saddles and keels and rostra of the Ammonoids, the straight internal shell 
with its peculiar structures and the guard of Belemnoids, and also the degenerate 
broad internal shell or pen of the Sepioids, are plainly of this nature. Effort 
working alone upon a common organism could, of course, not produce such re- 
sults. It is evident that it must have been assisted by the continuous action of 
physical surroundings. The law, therefore, as referred to in the Preface,’ seems 
to be, that, in so far as causes and habits are similar, they probably produce 
representation or morphological equivalence between different series or forms 
of the same type in the same habitat, and in so far as they are different, they 
probably produce the differentials which distinguish series and groups from 
each other. 
being an efficient cause in itself, or at least constitutes the last term in a series of causes. Hence Lamarck- 
ianism in a modern form, or, as we have termed it, Neolamarckianism, seems to us to be nearer the truth 
than Darwinism proper or ‘natural selection.” 
These words are so nearly our own views, and so valuable to us as confirmations of the theoretical 
results given in the text of this memoir, that we regret not being able to quote still more largely from 
this thoroughly scientific and philosophical writer. 
1 Page vi. No. 18. 
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