iS94-l O / b rPoiilton. 
The politician, Henry Fawcett. saw, long before scientific 
people themselves understood what Darwin meant by natural 
selection, that logically some result must ensue from such co-op- 
eration. Fawcett said that natural selection must produce evolu- 
tion as surely as a round stone will roll further than a square one. 
Some measure of evolution is simply the logical result of the 
co-operation of these three undisputed, abundantly proved factors. 
Now, some writers have thought to undermine the theory of 
natural selection by arguing that the important and essential 
factor of individual variation is not explained by the theory which 
rests upon it. True, it is not ; but for the theory of natural 
selection, the explanation does not signify. So long as individual 
variation is present, so long as it is hereditary, it does not signify 
how it is produced. There are, indeed, many theories professing 
to account for it ; but biologists are not generally agreed as to the 
manner in which it is produced. But so long as it is there, it is 
available, and natural selection can make use of it. 
It is interesting to note that, when Newton discovered the 
principle of universal gravitation, some people maintained that he 
had discovered nothing because he had not explained what gravity 
itself was. Now after two hundred years we can safely assert 
that universal gravitation stands out as one of the most trium- 
phant discoveries of the hiaman intellect ; and yet we, even now, 
are just as much in the dark as to what gravitation itself is as 
when Newton wrote. Exactly so it is with regard to individual 
variation. So long as it is a fact essential to organic nature, that 
one individual must be different from another, and so long as 
these differences are hereditary, so long may natural selection 
have abundant material for its work, even though it is unable to 
explain how that individual difference is produced. I am very 
far from undervaluing the interest of such an explanation ; on the 
contrary, I maintain that it forms one of the most interesting of 
biological problems now before the scientific world, or Hkel}^ to 
be before it for many a day. 
In fact, every successful attempt at scientific explanation only 
interprets down to a certain level of causation ; and this is just as 
true of universal gravitation and natural selection as it is of 
smaller efforts. Down to a certain level of causation, natural 
selection explains at any rate some part of organic evolntion. A 
more fundamental level would be to explain the factors upon 
