64 PROBLEMS OF EVOLUTION 



help regarding as a self-imposed burden, as I have already said. 1 

 I wish now to point out that the reproduction of lost parts 

 should be the bete noir of the Lamarckian rather than of the 

 Neo-Darwinian. For it is clear, when a lobster replaces a lost 

 claw, that nothing external is the cause of the re-growth ; the 

 loss has merely stimulated the organism to put forth a power 

 that belonged to it at birth, viz., the power of replacing a lost 

 limb. 



The mutilation of embryos brings out this power in a still 

 more marked way. When the egg of a sea urchin is in its 

 second stage, no longer a single cell, but two cells united, it has 

 been found that if the two components are separated, each cell 

 will develop as if no separation had taken place and will even 

 advance to the gastrula stage. What better evidence can we 

 have that no external influence shapes the embryo, but that it 

 develops according to its own laws ? It is true that some 

 experiments on the eggs of frogs have not given the same results : 

 when one cell., of two or four developed from the single original 

 cell, was killed (not separated), it was not replaced, but here the 

 conditions seem to have been unfavourable. 2 



The experiments on the sea urchin's eggs show how strong 

 and persistent is the character of the germ-plasm, how capable it 

 is of piloting itself. We must go beyond mere externals if we 

 wish to find the cause of variations. Weismann, indeed, con- 

 cluded that the germ-plasm might be influenced by external 

 conditions acting upon it directly, not indirectly through the 

 soma, but such influence I regard only as a stimulation to the 

 organism to vary, not as a cause of variations, since the growth 

 that results is often in no sense an adaptation to the conditions 

 that are said to have produced it. 3 



Selection There still remain difficulties to clear up. It is often urged 

 of food tnat tne germ-plasm depends entirely for its nourishment on the 

 individual, of which it forms part or in which it resides, and 

 that its character must depend largely on the diet which is 

 supplied to it. This argument looks formidable at first sight. 

 We must remember, though, that you may take a horse to the 



1 See pp. 53, 131. 2 See Weismann's Germ Plasm, pp. 136, 137. 3 See p. 53. 



