ually work up from the earliest stages of
development to the latest. Before starting, however, I ask the reader to
bear in mind one consideration, which must reasonably prevent our
anticipating that in _every case_ the life-history of an individual
organism should present a _full_ recapitulation of the life-history of
its ancestral line of species. Supposing the theory of evolution to be
true, it must follow that in many cases it would have been more or less
disadvantageous to a developing type that it should have been obliged to
reproduce in its individual representatives all the phases of
development previously undergone by its ancestry--even within the limits
of the same family. We can easily understand, for example, that the
waste of material required for building up the useless gills of the
embryonic salamanders is a waste which, sooner or later, is likely to be
done away with; so that the fact of its occurring at all is in itself
enough to show that the change from aquatic to terrestrial habits on the
part of this species must have been one of comparatively recent
occurrence. Now, in as far as it is detrimental to a developing type
that it should pass through any particular ancestral phases of
development, we may be sure that natural selection--or whatever other
adjustive causes we may suppose to have been at work in the adaptation
of organisms to their surroundings--will constantly seek to get rid of
this necessity, with the result, when successful, of dropping out the
detrimental phases. Thus the foreshortening of developmental history
which takes place in the individual lifetime may be expected often to
take place, not only in the way of condensation, but also in the way of
excision. Many pages of ancestral history may be recapitulated in the
paragraphs of embryonic development, while others may not be so much as
mentioned. And that this is the true explanation of what embryologists
term "direct" development--or of a more or less sudden leap from one
phase to another, without any appearance of intermediate phases--is
proved by the fact that in some cases both direct and indirect
development occur within the same group of organisms, some genera or
families having dropped out the intermediate phases which other genera
or families retain.

* * * * *

The argument from embryology must be taken to begin with the first
beginning of individual life in the ovum. And, in order to understa