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avroZcuov or complete type of animal. It consists of a 
certain definite number of organs, composed of a cei'tain 
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- 
