362 F. IV. HEADLEY [may 



their food and habits, approximates to that of placental mammals 

 of similar habits. The heart of a bird with its four chambers is 

 strangely like the heart of a mammal, and the difference between the 

 avian and mammalian valves between the two right-hand chambers 

 only serves to bring out the similarity of the main architectural 

 features. Here, then, we have two undoubted examples of convergence ; 

 several other highly probable instances have already been mentioned. 

 Is such convergence likely, if the number of possible variations is 

 unlimited ? In this connection much might be said on the subject of 

 what Darwin called " analogous variations," the phenomenon of two 

 allied species varying in the same way. 1 In some cases it may be 

 possible to see the actual limit of the capacity to vary. I cannot help 

 thinking that we know all the kinds of outgrowth of which the 

 epidermis is capable ; that hair, wool, bristles, teeth, the various 

 formations of horny material, scales, and feathers, are the only possi- 

 bilities. I infer this from the elasticity of the environment — a 

 subject to which I shall soon return — which allows an organism to 

 choose its own method of adaptation. I would set no limit to the 

 possible shapes and positions of such epidermic growths. I am 

 speaking only of the kinds of material available. 



Lastly, I w T ould call attention to the continuity of variation, to use 

 Darwin's term for it : the known likelihood that among the offspring 

 of parents in which a particular variation lias occurred some will vary 

 in the same direction, so that if a flower shows a tendency to double- 

 ness, some of the seedling plants from it are likely to show the same 

 tendency in a greater degree. There are two main principles, then, 

 on which I wish to insist — (1) that there is at any stage in evolution 

 a limit to the number of possible variations, (2) that there is con- 

 tinuity in variation. 



The establishment of the former of these two principles introduces 

 another difficulty. If any variation that appears in any animal must 

 be in conformity with the organisation of the animal in question, and 

 if the number of possible variations is thus limited, how is it that 

 adaptations to environment are as perfect as they are ? It is the con- 

 stant cry of Lamarckians that variations are all definite, i.e. not vague 

 tentative deviations, but unmistakable adaptations. I cannot myself 

 accept this proposition when thus broadly stated. But, recognising 

 that adaptation is the rule in nature, I wish to show how a considera- 

 tion of the character of the environment will help to remove the 

 difficulty. 



The environment offers to animals all that they require, and lets 

 them take what they want in any way that they may choose. They 

 must have oxygen, but they may get it by whatever means they like. 

 They must have protoplasm for food, but they may adopt any means 



1 See "Animals and Plants under Domestication," vol. ii. p. 348; see also Eimer's 

 " Zoologische Studien auf Capri." 



