« INTRODUCTION. 



of the surrounding physical conditions and by correlation of 

 growth. 



On the principles here briefly sketched out, there is no 

 innate or necessary tendency in each being to its own advance- 

 ment in the scale of organization. We are almost compelled 

 to look at the specialization or differentiation of parts or organs 

 for different functions as the best or even sole standard of 

 advancement ; for by such division of labour each function of 

 body and mind is better performed. And, as natural selection 

 acts exclusively through the preservation of profitable ^modifica- 

 tions of structure, and as the conditions of life in each area 

 generally become more and more complex, from the increasing 

 number of different forms which inhabit it and from most of 

 these forms acquiring a more and more perfect structure, 

 we may confidently believe, that, on the whole, organization 

 advances. Nevertheless a very simple form fitted for very simple 

 conditions of life might remain for indefinite ages unaltered or 

 unimproved; for what would it profit an infusorial animalcule, 

 for instance, or an intestinal worm, to become highly organized? 

 Members of a high group might even become, and this appa- 

 rently has occurred, fitted for simpler conditions of life ; and in 

 this case natural selection would tend to simplify or degrade 

 the organization, for complicated mechanism for simple actions 

 would be useless or even disadvantageous. 



In a second work, after treating of the Variation of organisms 

 in a state of nature, of the Struggle for Existence and the prin- 

 ciple of Natural Selection, I shall discuss the difficulties which are 

 opposed to the theory. These difficulties may be classed under 

 the following heads :— the apparent impossibility in some cases 

 of a very simple organ graduating by small steps into a highly 

 perfect organ; the marvellous facts of Instinct; the whole ques- 

 tion of Hybridity ; and, lastly, the absence, at the present time 

 and in our geological formations, of innumerable links connect- 

 ing all allied species. Although some of these difficulties are of 

 great weight, we shall see that many of them are explicable on 

 the theory of natural selection, and are otherwise inexplicable. 



In scientific investigations it is permitted to invent any hypo- 

 thesis, and if it explains various large and independent classes 

 of facts it rises to the rank of a well-grounded theory. The 



