136 Provisional Hypothesis of Saltatory Evolution. [Mareh, 
nature occasionally exhibits, may still be perfectly in accordance 
with the view that all change is by minute differences gradually 
accumulated in response to the environment, I would offer the 
following example : — 
In any sloping gutter of a paved street not too cleanly swept, 
every one will have noticed during a sudden shower how small 
particles of earth and other materials will sometimes act as a dam, 
producing a puddle which, relieved by partial drainage, may for 
a time appear to remain in statu quo. A time comes, however, 
when the gradually accumulated pressure suddenly sweeps the 
dam before it for a short distance. Then another similar pool is 
formed, and so on indefinitely. 
(2.) The modern idea of a species may be stated to be a 
greater or lesser number of similar individual organisms in which 
for the time being the majority of characters are in a condition of 
more or less stable equilibrium ; and which have the power to trans- 
mit these characters to their progeny with a tendency to maintain 
this equilibrium. 
(3.) This tendency may be in some cases sufficiently strong 
to resist for a considerable period the changes which a gradual 
modification of the environment may tend to bring about. When 
the latter has reached a pitch which renders the resistance no 
longer effectual, it is conceivable that a sudden change may take 
place in the constitution of the organism, rapidly adapting it 
once more to its surroundings, upon which the tendency to equi- 
librium may reassert itself in the minor characteristics, and these 
may, as it were, crystallize once more in a manner not dissimilar 
in its results to the form which was recognizable in the earlier 
generic type. 
(4.) If among a certain assemblage of individuals forming a 
species the tendency to maintain the specific equilibrium is (as 
it should be, a priori) transmitted to individual offspring in dif- 
ferent degrees of intensity, a gradual separation may take place 
between those with the stronger tendency to equilibrium, and 
those with less. 5 
(5.) Those yielding to the pressure of the environment (let us 
say in the manner indicated in paragraph 3) must by the law of 
natural selection become better adapted to it, and with their 
changed generic structure may be able to persist. 
(6.) On the other hand, those with the broader base, so to 
speak, with an inherited tendency to remain unshaken by the 
modifications of the environment, may be conceived as being and 
