8 INTRODUCTION. 



exists to the number, singularity, and perfection of the 

 contrivances and co-adaptations which may thus be pro- 

 duced. An animal or a plant may thus slowly become 

 related in its structure and habits in the most intricate 

 manner to many other animals and plants, and to the 

 physical conditions of its home. Variations in the organiza- 

 tion will in some cases be aided by habit, or by the use and 

 disuse of parts, and they will be governed by the direct 

 action 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 ad- 

 vancement 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 modications 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 apparently has often 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. 



The arguments opposed to the theory of Natural Selection, 

 have been discussed in my ' Origin of Species,' as far as the 

 size of that work permitted, under the following heads : 

 the difficulty in understanding how very simple organs have 

 been converted by small and graduated steps into highly 

 perfect and complex organs ; the marvellous facts of 



