i877-J an & ^ ie Evolution Hypothesis . 529 
The fadt already alluded to, that the development of an 
organism may be arrested either wholly or partially, and its 
growth continue as if no such arrest had taken place, has 
an important bearing on this subject, for it appears to show 
that development is distindt from the nutritive and repara- 
tive— in short, the purely mechanical processes of life — and 
that it is therefore due to an additional cause, the adtion of 
which may be suspended without arresting the ordinary 
physical processes. 
The correlative development of all parts of an organism 
already referred to evinces systematic evolution. The con- 
vergence of all types to one, and the existence at the present 
day of forms closely allied to others long extin (ft, appears 
to indicate that organisms admit of development or evolu- 
tion by certain invariable metamorphoses only. 
We have no longer, at the present day, to concern our- 
selves with establishing the Evolution Hypothesis. Almost 
all those who are in a position to form a judgment are 
agreed in accepting it. 
We must now confine ourselves to making a choice be- 
tween two theories in explanation of the dodtrine of descent. 
First , we may follow the Darwinians and believe that 
natural and sexual selection, habits, and other accidental 
circumstances are sufficient to account for all organic phe- 
nomena. If we maintain this, we are bound also to admit 
the purely mechanical view of life; because, in asserting 
the all-sufficiency of seledtion, &c., we preclude the adtion 
of all other agents with the exception of molecular force. 
Again, if we adopt the mechanical view, we must, to be 
consistent, consider natural seledtion, &c., adequate to ac- 
count for all forms of strudture and all organic changes, 
because we eliminate all other possible agents — crystalline 
force, from its very nature, being wholly blind, casual, acci- 
dental, and in every way purely fortuitous. Furthermore, 
those who hold mechanical causes, diredted by natural 
seledtion, &c., sufficient to account for the evolution of all 
animals and plants, are logically compelled to admit spon- 
taneous generation as a necessary consequence ; for other- 
wise there is imposed an arbitrary limitation to the extent 
or range of power of the molecular forces, and it is admitted 
that a first step is necessary, after which mechanical causes 
come into play — this is tantamount to acknowledging the 
existence of an agent separate and distindt from mechanical 
adtion. Thus the theory of seledtion, as sole agent of evolu- 
tion, the mechanical view of life, and the theory of spon- 
taneous generation, are inseparable ; they must stand or fall 
VOL. vii. (n.s.) z o 
