MR. a. J. ROMANES OK PHYSIOLOGICAL SELECTION. 405 



follow as a necessary contingency." So that, in view of tbese 

 considerations, I am disposed to think that even wholly negative 

 results yielded by this branch, of inquiry would not be absolutely 

 fatal to my theory, although, no doubt, most damaging to its 

 probability. 



The other branch of inquiry consists in looking out for cases 

 of two well-marked natural varieties living together on the same 

 area, and ascertaining by experiment whether these are not more 

 fertile within their own limits than they are with one another. 

 Plants would lend themselves to these experiments much more 

 readily than animals ; and in the case of plants the experi- 

 ments would not be very difficult to try, while the results when 

 obtained would be less open to doubt than those obtainable by 

 the method above mentioned. I therefore hope that botanists in 

 different parts of the world will deem it worth their while to see 

 whether it is not possible to gain this direct evidence, at once of 

 evolution as a fact, and of physiological selection as a method. 



The points to be attended to in conducting these experiments 

 are as follows. 



Let the varieties be well marked, or, at least, constant withiu 

 themselves ; let there be no question that both the varieties are 

 endemic as well as common to the area which they occupy. In 

 conducting the experiments care should be taken not to disturb 

 the natural conditions of the individuals chosen, whether by 

 transplantation or in any other way. And, of course, it is need- 

 less to add that not only care must be taken, but certainty 

 secured, that the only source of fertilization of the individuals 

 chosen is that of the pollen used by the observer. The experi- 

 ments, which ought to be conducted over a large number of indi- 

 viduals, will in every case divide themselves into four sets : — 

 1st, fertilization of A by B ; 2nd, fertilization of B by A ; 3rd, 

 of A by A ; and 4th, of B by B ; where A and B are the two 

 varieties in question. In every one experiment of these four 

 sets of experiments the seed which is yielded must be counted 

 and sown. When all the experiments are over, let it thus 

 be ascertained whether there is any diiference in the degrees of 

 fertility which have been yielded by experiments 1 and 2, and by 

 3 and 4 respectively. 



