i877‘] 
Evolution by Expansion . 
33 
From these considerations, while agreeing with Darwin 
that community of type indicates descent from a common 
ancestor, I consider that the type was not acquired by that an- 
cestor , but was inherent in it, and still is inherent in its progeny . 
Type itself undoubtedly also develops and changes, but 
without reference to the conditions of existence or extra- 
neous influences, and solely by expansion through age after 
age, expansion wrought by the mysterious inherent force 
performing its work in a set time with the exactness and 
regularity observable in all operations of Nature. 
Modification of type to suit the conditions of existence 
must be more variable than type itself, as the same typical 
characteristics are common to a vast number of species 
adapted to most diverse conditions of life. There are com- 
paratively few leading types, but an enormous variety of 
adaptations of each type. The type of an organism there- 
fore changes very slowly in comparison to the alterations in 
its structure due to extraneous causes. 
These conclusions, arrived at from the evidence of mor- 
phological phenomena, are borne out in a very striking 
manner by those of embryology, taken in connection with 
the faCts of palaeontology. 
A study of these subjects also reveals to us more fully the 
nature of an organism. 
The embryos or larvae of all animals pass through many 
phases in the course of development. At certain stages the 
embryos of reptiles, birds, and mammals so closely resemble 
one another as to be undistinguishable. The embryo of the 
lower organisation is, in this class of animals, arrested in its 
growth, or more properly development, at an earlier stage 
than that of the higher : that is to say, the unformed em- 
bryonic mass does not at once acquire the characteristics of 
its species, but first assumes the form of a remote progenitor 
of its class, and expands through progressive stages till it 
arrives at the degree of development of its parents. In 
these stages the embryo frequently resembles extinCt mem- 
bers of the class to which it belongs. In short, the embryo 
is developed by the same stages by which the race to which 
it belongs was developed in the course of successive geolo- 
gical eras ; i.e., the development-history of the individual 
is a recapitulation of the development-history of the stock 
to which the individual belongs. The various phases 
are so merged into one another, in the development 
or expansion during gestation, as somewhat to obliterate 
their characteristic traits and to render them indefinite, but 
still they indubitably accurately correspond to the stages of 
VOL. vii. (n.s.) v 
