320 THE CAUSES OF THE 



time when they each and all of them resemble 

 this one of the dog. But as development 

 advances, all the parts acquire their speciality, 

 till at length you have the embryo converted into 

 the form of the parent from which it started. So 

 that you see, this living animal, this horse, begins 

 its existence as a minute particle of nitrogenous 

 matter, which, being supplied with nutriment 

 (derived, as I have shown, from the inorganic 

 world), grows up according to the special type and 

 construction of its parents, works and undergoes a 

 constant waste, and that waste is made good by 

 nutriment derived from the inorganic world ; the 

 waste given off in this way being directly added 

 to the inorganic world. Eventually the animal 

 itself dies, and, by the process of decomposition, 

 its whole body is returned to those conditions 

 of inorganic matter in which its substance 

 originated. 



This, then, is that which is true of every living 

 form, from the lowest plant to the highest animal 

 to man himself. You might define the life of 

 every one in exactly the same terms as those 

 which I have now used ; the difference between 

 the highest and the lowest being simply in the 

 complexity of the developmental changes, the 

 variety of the structural forms, and the diversity 

 of the physiological functions which are exerted 

 by each. 



If I were to take an oak tree, as a specimen of 



