THEORETICAL BIOLOGY 469 



is a tendency for them to occupy a more and more extended 

 territor) ; some may even take to active migration into regions 

 where a greater food supply renders the struggle for existence 

 less severe, and thus the component individuals of the species 

 become separated into groups more or less completely isolated 

 from one another geographically. Or natural barriers may arise 

 through geological processes dividing a species into two or more 

 groups, which would thus be cut off from all communication 

 with one another. At times some members of a species may be 

 passively carried from one place to another, as on drifting logs 

 or in the case of some animals on the feet of birds. But in all 

 these various methods of distribution the ultimate result is that 

 some members of a species are isolated from the rest ; then 

 natural selection can effect the development of desirable varia- 

 tions and a new species be produced just as in the case of the 

 rabbits of Porto Santo already referred to. The faunae of many 

 isolated islands confirm this theory. While, however, it is 

 undoubtedly true that new species may arise in this way, yet 

 we are by no means sure that it is the only or even the most 

 usual method of species formation. 



We have then two theories of descent : the Lamarckian, which 

 attributes the origin of variation to the influence of the environ- 

 ment of the organism, — just how it does not say, — and develops 

 new species through the use and disuse of parts ; and the Dar- 

 winian, which does not attempt to account for variation, but 

 merely speaks of it as spontaneous, fortuitous, and accounts for 

 the formation of new species chiefly through the process oi 

 natural selection governed by the struggle for existence. It 

 will be noted, then, that we have no definite theory, up to this 

 point, of the origin of variation. A considerable advance was 

 made in the field of theoretical biology by the German scientist, 

 Weismann, in a series of essays on heredity beginning in [883. 

 Without o-oino- into the details of his theories here, we may 

 briefly note one or two points concerning the origin of varia- 

 tions. Nearly all animals develop from a single cell which is 

 formed by the very intimate fusion of two cells, the ovum and 

 the spermatozoon, which have developed in two individuals, the 

 female and the male; these two individuals differ, not only sex- 

 ually, but in many other characteristics as well. Now the ovum 



