ORIGIN OF VARIATIONS. 169 



still newer characters follow after from behind. 

 Thus during life, e.g. in lizards, a series of mark- 

 ings pass in succession over the body from behind 

 forwards, just as one wave follows another, and 

 the anterior ones vanish while new ones appear 

 behind. (Law of wavelike evolution, or law of 

 undulation.)"^ Even more striking is the follow- 

 ing quotation in the same connection: "Wurten- 

 berger finds that in ammonites all structural 

 changes show themselves first on the last (the 

 outer) whorl of the shell, — as in living animals, 

 e.g. in my lizards, at the tail, — and that then such 

 a change in the following generations is pushed 

 farther and farther towards the beginning, — as, 

 e.g. in my lizards, towards the head, — until it 

 prevails in the greater number of the whorls. . . . 

 Then still newer characters may arise again on 

 the most external whorl — just as in lizards at the 

 tail — and drive back the former, and so on. The 

 ammonites also only at an advanced age, only 

 when they have as exactly as possible gone through 

 the course of development inherited from their 

 parents, acquire the power to vary in a new direc- 

 tion ; but this power can be inherited in such a 

 way that in following generations it always ap- 

 pears a little earlier, until it itself characterises the 



1 Organic Evolution., Eimer. Trans, by Cunningham (Macmillan & Co.). 



