1898] 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 conies, " 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, 1 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 



