FEEDING OF ANIMALS. 353 



tion of increase in live-weight for a given amount of dry sub- 

 stance of the food will not increase with the increased pro- 

 portion of nitrogenous compounds consumed ; whilst, so far 

 as these are in excess, the proportion of carcass in the live- 

 weight will probably be somewhat less, and the carcasses 

 themselves will be somewhat more bony and fleshy, and less 

 fat. 



We at the same time pointed out, however, that the com- Estimat- 

 parative values of food-stuffs, even as such, could not be uncon- ^/Z^ 

 ditionally determined by the percentage of the total nitro- 

 genous and the total non-nitrogenous constituents ; that it 

 was necessary — to examine more closely into the nature and 

 condition of the proximate compounds of food-stuffs; to 

 distinguish those which are digestible and assimilable from 

 those which are not so ; to determine the relative values of 

 the comparable or mutually replacable portions ; and, finally, 

 to fix our standards of comparative value with more of refer- 

 ence to direct experimental evidence on the point, and to 

 existing knowledge of the composition of the animal bodies, 

 than had hitherto been usual or even possible. 



Since then, an immense amount of labour has been ex- 

 pended in the determination of the digestibility of the indi- 

 vidual constituents of various food-stuffs ; and the results so 

 far obtained form a valuable contribution to our information 

 on the subject. There is, however, wide variation in the 

 composition of different samples of nominally the same 

 description of food. Then, the determinations of the amounts 

 of the various constituents remaining undigested have gener- 

 ally been made with animals fed on limited supplies of food, 

 for maintenance only ; and the experiments have frequently 

 been made with the individual foods given separately. Great Xecessity 

 care and reservation are, therefore, necessary in the applica- ^oSJ n 

 tion of the results to actual practice. Thus, in the liberal estimates 

 feeding of animals for the production of increase, it is gener- °^/°°f 

 ally economical to give, within limits, an excess of food, if a 

 maximum result is to be obtained for a given live-weight of 

 animal within a given time ; and, in the case of animals liber- 

 ally fed for the exercise of force, there will also generally 

 be an excess of food given. It is obvious that, under the 

 conditions of actual practice here assumed, greater proportions 

 of the various constituents consumed will remain undigested 

 than would be indicated by the figures representing indiges- 

 tibility obtained under the usual conditions of experimenting 

 on the point above referred to. Then there is the important 

 consideration, that conclusive evidence is still wanting as to 

 the exact rdle in the system of some prominent constituents 

 of food-stuffs. For example, there is yet much uncertainty 



VOL. VII. z 



