METHODS OF STUDY 13 



under consideration. The resulting numbers are then 

 ranged in order so that a precise view of the numerical 

 characters of a large number of specimens can be 

 readily obtained. By the use of such methods valu- 

 able information is often to be arrived at. But the 

 same limitation affects them as in the preceding case. 

 So that the only way in which we can come to a 

 definite decision as to whether a given putative 

 species does or does not represent a definite and con- 

 stant type is by resorting to our third method, that 

 of sowing its seeds and actually rearing its progeny. 

 And this is not so simple a matter as might appear 

 at first sight, for a great many precautions have to 

 be taken. Thus we must separately sow the seeds of 

 many different individuals of the species which we 

 are examining, so as not to base our conclusions upon 

 a few experiments only. But in many cases, even 

 when this has been done we should only know one of 

 the parents of our seedlings that is to say, in cases 

 where the pollen for fertilization may possibly have 

 been conveyed by natural agencies from a different 

 plant. In such a case we must either ensure self- 

 pollination by isolating our plants, or we must arti- 

 ficially provide pollen from a separate known parent. 

 If under these circumstances a particular group of 

 plants preserves the characteristic differences which 

 distinguish it from another group which has also been 

 grown for a number of generations under the same 

 conditions* we have at last reasonable grounds upon 

 which to base the opinion that we are dealing with 

 two distinct physiological species^ even though the 



