1894- NOTES AND COMMENTS. 405 



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 equilibrium, and which have the power to transmit these 

 characters 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 modi- 

 fication 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 

 its specific results to that which was recognisable in the earlier 

 generic type. If among a certain assemblage of individuals con- 

 stituting a species, the tendency to maintain the specific equili- 

 brium be (as it should be a priori) transmitted to the progeny in 

 different degrees of intensity, a gradual separation 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 with the broader base, so to speak, with the inherited 

 tendency to remain unshaken by the modifications of the environment, 

 may be conceived as through this tendency to be and to remain less 

 injuriously affected by adverse circumstances, and consequently might 

 still endure. In short, natural selection in the one case would find 

 its fulcrum in the tendency to easy adjustment of characters ; and in 

 the other case in the inherited persistency in equilibrium rendering 

 its possessor more or less indifferent to the injurious elements of the 

 environment. The intermediate individuals by the hypothesis would 

 be those least-fitted to persist in any case and hence liable to be 

 rapidly eliminated. Then we should have parallel series of species 

 in two or even more genera existing simultaneously." 



Induced Variation. 



How far the variation of organisms can or does take place in 

 response to change of environment is one of those questions that the 

 biologist is bound to answer before he can solve the problem of species. 

 A determined attempt to answer this question has, we all know, 

 been made for many years by Professor E. B. Poulton, in his experi- 

 ments on caterpillars. At the Royal Society's conversazione, on 



