4 12 THE GERM-PLASM 



forces for the explanation of natural phenomena until we have 

 proved that the known ones are insufficient. 



Such a phyletic principle of development would naturally 

 not produce ordinary individual variations, but would lead to 

 modifications which affect all the individuals of a species in the 

 same manner. All the individual modifications which actually 

 exist would, in that case, have to be considered insignificant 

 in phyletic development ; and in plants Nageli actually 

 looked upon them to be transitory, and not hereditary habitat- 

 modifications (' Standorts-Modificationen'). But this view is 

 opposed by the transmission of an immense number of indi- 

 vidual differences in man and animals. 



If we recognise that the processes of selection are the chief 

 factors in organic development, we must attach the greatest 

 importance to these hereditary differences in individuals, and 

 must try to discover their origin. 



Brooks accepts this view, and has propounded a theory of 

 heredity which is based on Darwin's hypothesis of pangenesis, 

 and according to which variation is in the main dependent on 

 sexual reproduction.* Variability, in his opinion, is caused by 

 every ' gemmule ' of the spermatozoon uniting with ' that particle 

 of the ovum which is destined to give rise in the offspring to the 

 cell which corresponds to the one which produced the germ or 

 gemmule. . ' . When this cell becomes developed in the 

 body of the offspring // will be a hybrid, and it will therefore 

 tend to vary.'' Brooks, moreover, assigns different functions 

 to the two kinds of germ-cells, and represents them as being 

 charged or filled to different degrees with ' gemmules,' the 

 egg-cell containing a much smaller number than the sperm-cell. 

 In his opinion the egg-cell is the conservative principle which 

 controls the transmission of purely racial or specific characters, 

 whereas the sperm-cell is the progressive element which causes 

 variation. 



Brooks has ingeniously advanced every argument which could 

 be brought forward in favour of his theory, but I doubt whether 

 he still holds to it, for many new discoveries have since been 

 made which contradict it. Although the view that 'acquired' 

 characters are not hereditary is not universally admitted, it has 



* W. A. Brooks, ' The Law of Heredity : A Study of the Cause of Varia- 

 tion and the Origin of Living Organisms,' Baltimore, 1883. 



