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avToZwov or complete type of animal. It consists of a 

 certain definite number of organs, composed of a certain 

 definite number of parts. It will also have certain aliments, 

 location, enemies, &c., which we may call its province? 

 necessary for its life. Thus our type animal is capable of a 

 flux passing through all possible forms and provinces in all 

 possible combinations. I include amongst these of course 

 many arrangements necessarily absurd. To each arrangement 

 of organs and provinces thus imagined would correspond a 

 certain vitality or power of living in the type. I mean not 

 merely power of individual existence, but existence as a 

 race. 



The vitality is therefore a function of a large number of 

 variables, some independent, others connected by equations 

 of condition. It is to us quite an unknown function, but 

 not therefore indefinite. Therefore, as in any other function 

 of variables, certain relations amongst the variables will 

 give maxima values of the vitality. These maxima of 

 vitality constitute species. Vitality is not mere physical 

 might or agility or fecundity, but compounded of all. 



Now for a maximum, we know that any change in the 

 variables lessens the function. We thus see how species 

 are stable. In the constant variation, for no being seems 

 capable of reproducing itself exactly, all individuals have 

 less vitality as they depart from the special type which 

 gives the maximum of vitality, and will be choked out by 

 those which, being nearer to the type, possess more vitality. 

 So Hybrids, intermediate between two maxima, will possess 

 less vitality than either, and will be choked out, though 

 the main cause of failure is that the process is like that 

 devised by Swift's Laputan philosopher, who sawed the 

 Whigs' and Tories' heads in half, and changing them, left each 

 brain to settle its politics in itself. So the poor mule, 

 with a bundle of habits, half horse and half ass, in this 

 intestine conflict, has little power to take care of itself 

 Of course all maxima may not have plants or animals repre- 



