Originative Factors in Evolution. By J. A. Thomson. -447 



suppose that a change in the rate of a particular kind of metabolism 

 may reverberate through the body. As Mr. J. T. Cunningham 

 and Professor Dendy have pointed out, an augmentation or a 

 diminution of certain internal secretions or hormones might have 

 multitudinous transforming effects. 



§ 9. The growing body of evidence that mutations or brusque 

 variations are not infrequent in their occurrence, and that if they 

 come they often come to stay, lessens the element of tlie casual in 

 organic evolution. It also lessens the need for over-burdening the 

 role of natural selection, especially as an accumulator of minute 

 increments or decrements. But along with this there should be 

 considered the idea that variations are limited in some measure by 

 what has gone before. At the- beginning of each individual life 

 there is the fertilized ovum, a viable unity. If a variation occur 

 it is not like to grip unless it be congruent with the germinal 

 organization already established ; it must harmonize, just as an 

 addition to a crystal must, but within a wider range. The 

 character of the buildino- that has been erected determines in some 

 measure the nature of an addition to it. The idea of architecture 

 is, of course, only one aspect ; the novelty must be congruent with 

 the previously established reaction-system and specific metabolism. 

 Out of the same spring we do not get sweet water and bitter. 

 Thus the element of the fortuitous shrinks still farther. It is 

 interesting to find that monsters sometimes result from infelicitous 

 crossings, but perhaps a greater interest attaches to the fact that 

 monsters are so rare in nature, not only in survival, Ijut in 

 occurrence. 



An illustration of the limiting of changes by pre-existing 

 organization may be found in a recent paper * by Professor S. J. 

 Hickson, in which he notes that meristic variability in important 

 organs is much greater in radially symmetrical forms than in 

 bilaterally symmetrical forms where a balance must be kept. In 

 reference to the Pennatulacea he shows that variable or plastic 

 characters may become less variable or plastic as a transition is 

 made from radial to bilateral symmetry, and points out that 

 increasing rigidity of certain characters leads in some cases to the 

 differentiation of the discontinuous groups which are recognized 

 as species. What we w^ould suggest is carrying this idea from the 

 fully-formed organism to the germ-cell organism, from the pheno- 

 type to the genotype, and considering substantive as well as 

 meristic variations. 



§ 10. Let us sum up. Germinal disturbances or re-arrangements 

 occur, and these may find expression in development as variations 

 or mutations of the organism. The question is, What brings about 

 the re-arrangements ? a question to be asked in the light of the 



* Mem. and Proc. Manchester Lit. and Phil. Soc, Ix. (1916) pp. 1-15. 



