CAUSES OF VARIATION. 3 



percentage of the food voided in the solid excreta 

 varies in the case of different animals. 1 



Another consideration which enters into the ques- 

 tion is the age, as well as the treatment, of the animal. 

 A young animal, during the period of its growth, 

 absorbs from its food into its system a larger quantity 

 of the three fertilising substances, nitrogen, phos- 

 phoric acid, and potash, than is the case with an adult 

 animal whose weight is neither increasing nor dimin- 

 ishing. A working horse similarly will return more 

 of the nitrogen, phosphates, and potash in its dung 

 than one not at work, and which is permitted to gain 

 in weight. The nature of the composition of the 

 solid excreta, therefore, will depend on the nature of 

 the food, age, breed, condition, and treatment of the 

 animal. 



Let us now investigate shortly the influence of the 

 above considerations. The solid excrements of the 

 common farm animals are generally distinguished from 

 one another according to the rate at which they de- 

 compose or ferment on keeping. Thus horse-dung is 

 generally known as a " hot " dung, while cow-dung, on 

 the other hand, is known as " cool/' Why this should 

 be so is not absolutely clear. Probably it is owing to 

 the fact that the former contains less water, as well 

 as (and this probably has more to do with it) to the 

 fact that it contains a larger percentage of fertilising 

 matter, especially nitrogen, thus affording conditions 



1 See Appendix, Note I., p. 53. 



