TOWER'S EXPERIMENTS 377 



to, a cross are important factors in determining the results 

 thereof. 



Tower's general conception of the attributes of the organism 

 appears to us to have a wholesome breadth and elasticity 

 such as the present state of knowledge demands. He recognises 

 the following facts as regards organic constitution : 



" i. That there is in organisms a form basis, relatively un- 

 alterable as regards symmetry, pattern and arrangement of parts. 



"2. That there are in organisms an array of attributes capable 

 of variation, but blending in heredity, forming blends and inter- 

 mediates. 



"3. That there are in organisms an array of attributes which 

 can exist only in a definite state of stability they are either 

 there or not there. 



" 4. That there are in organisms characters that by crossing 

 can be replaced by other more or less similar but different 

 characters." 



Johannsen's Experiments on Pure Lines. Experiments by 

 Nilsson and others at Svalof in Sweden have shown that the 

 progeny of an isolated particularly good ear of barley may all 

 exhibit the parental characters, and that their progeny in turn 

 breed true. If a single plant exhibits a desired result it is 

 shortest and surest to work from it alone, without going on for 

 years selecting also the nearest approximations. We owe to 

 the Danish botanist Professor Johannsen an elaboration of this 

 idea in a series of very important experiments, carried out 

 with unsurpassed patience and precision. He calls all the de- 

 scendants of a single individual in a self- fertilising race a "pure 

 line." An apparently homogeneous race or " population " is 

 a congeries of pure lines. Given an isolated pure line, the 

 cultivator can get no more out of it ; there are plus and minus 

 " fluctuations," but even with selection there is always a return 

 to the average. Similarly, in a population, which is made up 

 of a congeries of pure lines, selection cannot do more than 



