THE FACTORS OF EVOLUTION 201 



of the sex-products. It is true that the spore produces the 

 prothallus as before, but this does no longer produce sex-organs, 

 but proceeds, contrary to all rules, at once to the formation of 

 spores. 



It seems now clear that the evolution of the organism is 

 strictly dependent upon numerous external factors. We have 

 seen that organisms are not unalterable quantities, but that 

 there exists correlation between them and surrounding nature. 

 Alterations of the conditions of life immediately call forth more 

 or less radical alteration in the characters of the species. 



It is a question of another kind whether the change thus 

 produced may be regarded as an adaptation, or, in other words, 

 whether the organism responds to each stimulus by a suitable 

 modification. If we recall the different changes that took place 

 in Polygonum amphibium in water, on wet land, and on the 

 dunes it would seem as if we had here to deal with conscious 

 reactions. Many other facts may be interpreted in this sense. 

 But if, for instance, a bullfinch assumes a black colour on a diet 

 of hemp-seed, or an oyster, transplanted from the North Sea into 

 the Mediterranean, forms on its shell rich ornaments and long 

 spines ; or if, finally, the egg of the Sea-urchin, forced to develop 

 in water poor in lime, produces a Pluteus which has no support- 

 ing skeleton or arms, we are unable to perceive the purpose 

 in such changes. Indeed, many changes are obviously injurious 

 and weaken the vitality of the organism concerned. We see 

 therefore that direct influence produces useful as well as 

 indifferent and injurious alterations. 



Each organism maintains, as we have already seen, an ever- 

 changing balance with its environment. If the equilibrium 

 is disturbed by a change in the external conditions in life, the 

 organism either perishes or continues to modify until a new 

 state of equilibrium has been created. But this new state need 

 not necessarily be an advance upon the former, but only a more 

 or less successful ' squaring ' with the changed conditions of 

 life. It is exactly upon this that the constancy of the new 

 form depends. If the acquired modifications are useless or 

 indifferent, the individual concerned may be able to maintain 

 itself; but if they are injurious we are justified in assuming 

 that natural selection steps in and extinguishes the useless 

 organism. 



