PALEONTOLOGY 83 



difficult to apply, on account of our ignorance. 



One thing is evident. There is no general law, 

 either for development or for extinction, and, con- 

 sequently, there can be no general organic principle 

 regulating the length of duration of groups, as there 

 is with individuals. An analogy between the two 

 cases may sometimes be traced, but it is very incom- 

 plete, and becomes misleading if treated as more 

 than an analogy. 



Nevertheless, Professor Cope has put forward an 

 hypothesis, which he calls " Expression Points," 14 

 which is founded on this analogy, and which I con- 

 fess I find hard to follow. If I understand his 

 argument rightly, it is this. He commences by 

 saying that, during the development of every indi- 

 vidual there are one or more periods when develop- 

 ment is more rapid than at other times. He then 

 makes the statement that if reproduction takes place 

 during one of the rapid periods of development , the 

 offspring will be highly variable ; but if reproduction 

 takes place during one of the slow periods of develop- 

 ment, then the offspring will not be variable. And, 

 finally, he declares that it is the same with a genus 

 as with an individual. The genus also has periods 

 of rapid change, and periods of persistency; and 

 species originating when the genus is undergoing 

 rapid change will be variable, while those originating 

 during periods of persistency will be constant. 



Even if there were facts to support the statements 

 that offspring will be variable or not, according as 



14 '-Origin of the Fittest," p. 25 (1892). and "Primary 

 Factors of Organic Evolution," p. 25 (1898). 



