TRACHEATES 225 



are all grounded on precarious conjectures, not on facts. 

 Hence the chief postulate of the Lamarckian principle 

 itself requires explanation. Then there are the 

 numerous categories of facts in which the principle 

 entirely fails. There are so many characters of animals 

 that have never been exercised because they only act 

 when they are already there — such as colouring. How 

 could any conceivable exercise of the salivary glands 

 bring about the poisonous character of many serpents ? 

 Use can at the most cause the glands to secrete a 

 larger quantity, but not a different product ? And how 

 could the spines of the hedgehog arise from its being 

 often bitten in the skin by its enemies ? The thorns 

 that protect plants from being eaten are also just as 

 impossible to explain as due to use. 



We must, in fact, be very careful with the word use 

 or exercise. We often read in Lamarckian literature 

 that pressure will in one case lead to a less development 

 of the parts pressed, and in another case to a stronger 

 development.^ The first postulate of the theory is 

 therefore unsafe in many cases. It lends itself far too 

 much to being used as a mere phrase that explains 

 nothing. This organ must have been strengthened by 

 exercise, they say, knowing neither if it was in reality 

 much used, nor if exercise would modify it in the 



1 Thus in Plate's works pressure generally has a strengthening effect, 

 but in the case of the hermit-crab, the abdomen of which is buried in 

 a snail-shell, the steady pressure is regarded as having a weakening 

 effect and leading to the disappearance of its coat at this part. In 

 general, however, Plate makes a moderate and judicious use of the 

 Lamarckian principle. He recognises its weak points, but thinks it 

 is necessary for the explanation of certain phenomena. 



p 



