26^ SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



plants \ Sufficient justification for this opinion cannot, how- 

 ever, be furnished by the fact that in plants many characters 

 have not been as yet explained by adaptation. We should do 

 well to remember the extent to which the number of so-called 

 'morphological' characters in plants has been lessened during 

 the last twenty years. What a flood of light was thrown upon 

 the forms and colours of flowers, so often curious and ap- 

 parently arbitrary, when Sprengel's long-neglected discovery 

 was extended and duly appreciated as the result of Darwin's 

 investigations, and when the subject was further advanced by 

 Hermann Mailer's admirable researches ! Even the venation 

 of leaves, which was formerly considered to be entirely without 

 significance, has been shown to possess a high biological value 

 by the ingenious investigations of J. Sachs (see Appendix III, 

 p. 317). We have not yet reached the limits of investigation, 

 and no reason can be assigned for the belief that we shall not 

 some day receive an explanation of characters which are now 

 unintelligible'-. 



It is obvious that the zoologist cannot lay too much stress 

 upon the intimate connexion between form and function, a 

 connexion which extends to the minutest details : it is almost 

 impossible to insist too much upon the perfect manner in which 

 adaptation to certain conditions of life is carried out in the 

 animal body. In the animal body we find nothing without a 

 meaning, nothing which might be otherwise ; each organ, even 

 each cell or part of a cell is, as it were, tuned for the special 

 part it has to perform in relation to the surroundings. 



It is true that we are as yet unable to explain the adaptive 

 character of every structure in any single species, but whenever 

 we succeed in making out the significance of a structure, it 

 always proves to be a fresh example of adaptation. Any one 

 who has attempted to study the structure of a species in detail, 

 and to account for the relation of its parts to the functions of 

 the whole, will be altogether inclined to believe with me that 

 everything depends upon adaptation. There is no part of the 



^ 1. c. Preface, p. vi. 



' Since the above was written many other morphological peculiarities 

 of plants have been rightly explained as adaptations. Compare, for 

 instance, the investigation of Stahl on the means by which plants protect 

 themselves against the attacks of snails and slugs U^"'^} 1888^. — A. W., 

 1888. 



