OTHER THEORIES OF SPECIES-FORMING. 331 



exemplifies as follows : "In a sloping gutter of a paved 

 street not too cleanly swept every one will notice on a sudden 

 shower how small particles of earth and other materials 

 will sometimes act as a dam, producing a puddle which, 

 relieved by partial draining, may for a time remain in 

 statu quo. A time comes, however, when the gradually 

 accumulated pressure suddenly sweeps the dam before it 

 for a short distance, until another similar one is formed, the 

 pool again appears for a time to remain unchanged, and 

 so on indefinitely. Now 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 equi- 

 librium, and which have the power to transmit these char- 

 acters to their progeny with a tendency /to maintain this 

 equilibrium. 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 arrangement of the 

 constitution of the organism, adapting it once more to its 

 surroundings, when the tendency to equilibrium may reassert 

 itself in the minor characteristics, and they may, as it were, 

 crystallise once more in a form not dissimilar in generic 

 type. If among a certain assemblage of individuals con- 

 stituting a species, the tendency to maintain the specific 

 equilibrium be (as it should be a priori) transmitted to the 

 progeny in different degrees of intensity, a gradual separa- 

 tion might take place between those with a stronger 

 tendency to equilibrium and those with less. Here natural 

 selection would come in. Those yielding as above to the 

 pressure of the environment would necessarily become better 

 adapted to it (or perish) and with their changed generic 

 structure might be able to persist. On the other hand, those 



