340 EVOLUTION [CHAP. V 



Letter 253 if it may be so called, as the thoroughness and worth of my 



work entitles me to 



The collections which I have studied, it will be remembered, 

 are fossils collected without special reference to the very 

 minute subdivisions, such as the subdivisions of the Lower or 

 Middle Lias as made by the German authors, especially 

 Ouenstedt and Oppel, but pretty well defined for the larger 

 divisions in which the species are also well defined. The 

 condition of the collections as regards names, etc., was chaotic, 

 localities alone, with some few exceptions, accurate. To put 

 this in order they were first arranged according to their adult 

 characteristics. This proving unsatisfactory, I determined to 

 test thoroughly the theory of evolution by following out the 

 developmental history of each species and placing them within 

 their formations, Middle or Upper Lias, Oolite or so, according 

 to the extent to which they represented each other's charac- 

 teristics. Thus an adult of simple structure being taken as 

 the starting-point which we will call a, another species which 

 was a in its young stage and became b in the adult was placed 

 above it in the zoological series. By this process I presently 

 found that a y then a b and a b c, c representing the adult stage, 

 were very often found ; but that practically after passing 

 these two or three stages it did not often happen that a species 

 was found which was a b c in the young and then became d 

 in the adult. But on the other hand I very frequently found 

 one which, while it was a in the young, skipped the stages 

 b and c and became d while still quite young. Then some- 

 times, though more rarely, a species would be found belonging 

 to the same series, which would be a in the young and with a 

 very faint and fleeting resemblance to d at a later stage, pass 

 immediately while still quite young to the more advanced 

 characteristics represented by e, and hold these as its specific 

 characteristics until old age destroyed them. This skipping 

 is the highest exemplification, or rather manifestation, of 

 acceleration in development. In alluding to the history of 

 diseases and inheritance of characteristics, you in your Origin 

 of Species allude to the ordinary manifestation of acceleration, 

 when you speak of the tendency of diseases or characteristics 

 to appear at younger periods in the life of the child than of 

 its parents. This, according to my observations, is a law, or 

 rather mode, of development, which is applicable to all 



