104 



PROF. ERNEST W. MACBRIDE, M.A.. F.R.S.. OH 



according to the view generally held, consisted of single cells. 

 In such organisms, according to AVeisniann, it was possible for 

 acquired qualities, <; fluctuations," to be inherited, and the action 

 of different environments caused differences in hereditary poten- 

 tiality, out of which by varied combination the qualities of the 

 higher animals were built up. In resorting to this explanation 

 Weismann virtually gives up his case. There is no ground 

 whatever for the supposition that the simple animals are 

 constitutionally unlike more complex animals, and moreover all 

 the direct evidence which has been brought forward to support 

 the view that fluctuations are non-inheritable., applies with just 

 as much force to unicellular as to multicellular animals : in fact 

 some of the best evidence has been supplied by the study of 

 unicellular animals, and to this evidence we must now apply 

 ourselves. 



Whilst in the case of the majority of the higher animals the pro- 

 duction of young is impossible without the previous coalescence 

 in sexual union of two germs, which are carried by distinc: 

 individuals, yet this is by no means universally the case. There 

 are many cases in animals where both kinds of germs are borne 

 by the same individuals, which are then termed hermaphrodites. 

 and in this case the production of young by self-fertilization is 

 possible, and then we need not fear the introduction of 

 extraneous factors. Self-fertilization is possible in the case o: 

 the great majority of the higher plants. 



In other cases the egg is capable of developing without 

 fertilization, a phenomenon which is known as parthenogeiiesis ; 

 in this case also nothing but the hereditary potentiality of one 

 kind of parent need be considered. Lastly, in the lower animals 

 there is no distinction between body and germ-cell, but the 

 mother gives rise to two daughters by dividing in two, and for 

 long periods this kind of reproduction can go on without the 

 intervention of anything that could be called sexual union. 



In all three cases we have the opportunity of raising what has 

 been called a " pure line " of progeuy. For the case of self-fertili- 

 zation such a line has been investigated by Johannsen in the case 

 of the bean. Johannsen observed that if beans of a certain type 

 were taken, and individual beans sorted according to their weight, 

 a typical curve of error could be obtained, and if the larger 

 beans were selected the average size of their progeny was larger 

 than that of the smaller beans, though not so much larger as it 

 ought to have been in proportion to the size of the parents. 

 This want of proportionate increase was detected by Sir Francis 

 Galton and called by him "regression towards mediocrity." 



