404 POPULAR SCIENCE MONTHLY. 



character appears before or after the period of active reproduction.) 

 I should be inclined to attribute the character in both your cases to 

 the laws of growth and descent, secondarily to Natural Selection. It 

 has been an error on my part, and a misfortune to me, that I did not 

 largely discuss what I mean by laws of growth at an early period in 

 some of my books. I have said something on this head in two new 

 chapters in the last edition of the Origin. I should be happy to send 

 you a copy of this edition, if you do not possess it and care to have it. 

 A man in extreme old age differs much from a young man, and I pre- 

 sume every one would account for this by failing powers of growth. 

 On the other hand the skulls of some mammals go on altering during 

 maturity into advancing years; as do the horns of the stag, the tail- 

 feathers of some birds, the size of fishes, etc. ; and all such differences 

 I should attribute simply to the laws of growth, as long as full vigour 

 was retained. Endless other changes of structure in successive species 

 may, I believe, be accounted for by various complex laws of growth. 

 Now, any change of character thus induced with advancing years 

 in the individual might easily be inherited at an earlier age than that 

 at which it first supervened, and thus become characteristic of the 

 mature species; or again, such changes would be apt to follow from 

 variation, independently of inheritance, under proper conditions. 

 Therefore I should expect that characters of this kind would often 

 appear in later-formed species without the aid of Natural Selection, or 

 with its aid if the characters were of any advantage. The longer I 

 live, the more I become convinced how ignorant we are of the extent 

 to which all sorts of structures are serviceable to each species. But 

 that characters supervening during maturity in one species should 

 appear so regularly, as you state to be the case, in succeeding species, 

 seems to me very surprising and inexplicable. 



With respect to degradation in species towards the close of a series, 

 I have nothing to say, except that before I arrived at the end of your 

 letter, it occurred to me that the earlier and simpler ammonites must 

 have been well adapted to their conditions, and that when the species 

 were verging towards extinction (owing probably to the presence of 

 some more successful competitors) they would naturally become re- 

 adapted to simpler conditions. Before I had read your final remarks 

 I thought also that unfavourable conditions might cause, through the 

 law of growth, aided perhaps by reversion, degradation of character. 

 No doubt many new laws remain to be discovered. Permit me to add 

 that I have never been so foolish as to imagine that I have succeeded 

 in doing more than to lay down some of the broad outlines of the 

 origin of species. 



After long reflection I cannot avoid the conviction that no innate 

 tendency to progressive development exists, as is now held by so many 



