DISEASES OF THE OX AND SHEEP. 637 



and the preservation of life. We shall ever be on the alert to 

 find and to trace them, and then to strive with all the powers of 

 our minds to comply with their reiterated and most definite 

 injunctions. 



Now, the aspects from which life can be regarded are many and 

 various ; but one of the most instructive questions which are to 

 be decided is that to the consideration of which, after this intro- 

 duction or digression, we now come. It is, " How do the 

 different living things upon this earth reproduce themselves?" 

 That all living beings do, almost without exception, in the 

 ordinary state of healthful activity, reproduce their kind, is one 

 of the main points whereby they are characterised. By means 

 of this function each race is continued on in its existence from 

 generation to generation, until, in the lapse of ages, it is either 

 gradually developed and transformed into a successively more 

 and more highly endowed race, or, on the contrary, is perchance 

 rendered less and less able to exist under the new, and changed, 

 and ever changing, conditions, to which it is subjected. 



It has been shown that some kinds of organisms may be, so 

 to say, stamped out by reason of the inexorable course of that 

 incessant and interminable struggle for existence, whereby, as 

 we cannot but hope, the highest possible perfection will in the 

 far distant future be reached. In this connection it is to be 

 borne in mind that the same far-off result can, in so far as 

 animals are concerned, be in no small degree anticipated by the 

 judicious selection and management of the best and healthiest 

 stock on the part of the agriculturist, who may thus, by the 

 long-continued exercise of great care and judgment, gradually 

 but yet most surely improve his breeds of domesticated animals 

 to a truly wonderful extent, as indeed has been repeatedly and 

 most abundantly proved. 



If it is true, as we have alleged it is, that great difficulty may 

 be experienced in clearly damarcating in words the lowest 

 members of the animate kingdom from the more complex of the 

 aggregates occurring in the inanimate world, it is also most 

 noteworthy that difficulties of a like nature present themselves 

 in regard to marking off the many divisions made betwixt 

 different groups of living things themselves. Searching down 

 among the simplest living organisms by the aid of the strongest 

 magnifying powers of the microscope, we find that no constant 



