1s98) SCIENTIFIC PROOF v. ‘ A PRIORI’ ASSUMPTION 107 
stinging nettles. Cows often eat them, and man makes ‘ soup’ and 
‘spinach’ of them. 
Natural selection, being a purely imaginary agent, is as easy 
to manipulate as is the Creator’s name to account for phenomena, 
where no proof can be given. As soon as one asks for some 
grounds for such inferences, the retort comes, “‘In the present state 
of our knowledge, it is admitted that there are none!” 
It may be now desirable to state what scientific proofs consist of. 
There are two lines of evidence possible in support of some 
deduction arrived at for the interpretation of some natural pheno- 
menon. 
The first and best is experimental verification. If you find 
the result comes as you expected when you have supplied the 
conditions which, according to your deduction, you supposed to be 
capable of producing it, then that is all-sufficient and proves your 
theory to be fact. 
Take the case of spinous plants. One first observes as a 
matter of fact that spiny processes are particularly common in 
plants growing in arid soils and a dry atmosphere, whereas they do 
not appear among marsh or aquatic plants. It is always coincidences 
that one first looks for. Then the question arises, Is the spiny 
structure in any way due to these external conditions of the environ- 
ment? Now the test is to grow normally spiny plants in a good soil 
with plenty of moisture and in a moist atmosphere. Then follows 
the anticipated result that spines are no longer produced. If they 
be branch-spines, then the branches grow out into leafy shoots. If 
they be reduced and spinescent leaves, as in barberry, they at once 
develop into true leaves. To be quite sure you test it with other 
plants, and the same result follows. Your theory, therefore, is a 
proved fact, which henceforth is recognisable as an _ established 
natural law. 
A different line of proof is required when a deduction cannot be 
verified by experiment. It must then be established by induction, 
or the accumulation of probabilities in its favour, until the converse 
is practically unthinkable. This is the chief line of evidence 
for establishing evolution as set against the old form of natural 
theology and teleology. 
That I may not lay myself open to the charge of propounding 
what I have not done myself, I will take my deduction from an 
observation made in 1870, that irregular flowers are the result 
of the mechanical action of insects visiting them for honey or 
pollen. This conception cannot be proved experimentally, as it 
is impossible to make a regular flower become an irregular one. 
Another deduction has been drawn by others, namely, that gravity 
has been the cause of the enlarged lower petal or lip. The 
