xxvm] 'LAW OF IRREVERSIBILITY ' 347 



Some time after the present writer had deduced the Law of 

 Loss from a consideration of the structure of the water plants 

 living to-day, she learned that zoologists had already arrived, on 

 fossil evidence, at very similar conclusions regarding animals. 

 The Law of Loss covers part of the same ground as Dollo's 'Law 

 of Irreversibility.' That this law should have been arrived at 

 independently for plants and for animals is perhaps an indica- 

 tion of its probable validity. 



With current Mendelian conceptions, the ' Law of Loss ' 

 harmonises without apparent difficulty. If evolution has pro- 

 ceeded by variations due to successive losses of factors, we 

 should certainly expect that the complete loss of an organ might 

 be associated with inability to recall it, even when circum- 

 stances seem to put a premium upon its reappearance. 



If we accept the views of Samuel Butler so far as to admit that 

 there is at least an analogy of a highly intimate nature between 

 heredity and unconscious memory, each example of the ' Law 

 of Loss ' may perhaps be visualised as representing a lapse or 

 failure of memory. If an organ be lost, the remembrance of it 

 presumably in course of time becomes more and more remote, 

 until finally, even if circumstances renew the need for it, the 

 memory has so entirely faded that the plant cannot, as it were, 

 recall how to reconstruct it. It is thrown, so to speak, on its 

 own resources, and is thus compelled to discover for itself some 

 method of responding upon new lines to the ancient need. 



