UNITY OF PLAN IN DIVERSIFIED TYPES. 23 



modified are only of secondary importance in the life of 

 animals and plants, and that neither the plan of their 

 structure nor the various complications of that structure 

 are ever affected by such influences. What, indeed, are 

 the parts of the body which are in any way affected by 

 external influences 1 Chiefly those which are in imme- 

 diate contact with the external world, such as the skin, 

 and in the skin chiefly its outer layers, its colour, the thick- 

 ness of the fur, the colour of the hair, the feathers, and the 

 scales ; then the size of the body and its weight, as far as 

 it is dependent on the quality and quantity of the food ; 

 the thickness of the shell of Mollusks, when they live in 

 waters or upon a soil containing more or less limestone, 

 etc. The rapidity or slowness of the growth is also influ- 

 enced in a measure by the course of the seasons in dif- 

 ferent years ; so are also the fecundity, the duration of 

 life, etc. But all this has nothing to do with the essential 

 characteristics of animals. 



A book has yet to be written upon the independence 

 of organized beings of physical causes ; for most of what 

 is generally ascribed to the influence of physical agents 

 upon organized beings ought to be considered as a con- 

 nexion, established between them, in the general plan of 

 the creation. 



SECTION IV. 



UNITY OF PLAN IN OTHERWISE HIGHLY DIVERSIFIED TYPES. 



Nothing is more striking, throughout the animal and 

 vegetable kingdoms, than the unity of plan in the struc- 

 ture of the most diversified types. From pole to pole, in 

 every longitude, mammalia, birds, reptiles and fishes ex- 



