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University of California. 



[Vol. u 



change which took place here. Passing from the one era to the 

 other is like going from one world into another. How shall we 

 account for this great and apparently sudden change? 



Lost Records. — The first and greatest cause, especially of the 

 suddenness, is, of course, the loss of record. Certain leaves of the 

 book of time are missing, and when we come to read again, the 

 subject matter of the volume is greatly changed. This accounts 

 for the absolute suddenness in some cases, but not for the greatness 

 of the change, for this is out of all proportion to the lost interval 

 of time. Therefore we must admit farther: 



Rapid Steps in Evolution. — The steps of evolution were excep- 

 tionally rapid at this time. We have already given the causes of 

 this more rapid movement. But even when the record is recovered, 

 as for example, in India, Australia, etc., we find still that connecting 

 links between successive forms are rare — links are usually missing. 

 Therefore it is necessary to remember also that there are 



Feiv Generations Represented. — The changes being more than 

 usually rapid, the number of generations necessary to accomplish 

 a certain amount of change, i. e., to transform one species into 

 another, were comparatively few. Therefore when we remember 

 that fossils are but a small fraction of the actual number of individ- 

 uals living at any time, the likelihood of finding transitional forms 

 is diminished to that extent. Again, add to this the fact of 



Few Individuals in Each Generation. — The conditions must have 

 been unfavorable to fullness of life. The changes in the environment 

 were rapid, and therefore adaptation of organisms to the environ- 

 ment must have been imperfect. In a word, critical periods are 

 hard times. But hard times react and produce less reproductive fer- 

 tility, and therefore fewer individuals in each generation. Therefore 

 critical periods are characterized by poverty of fossils, even when 

 the record is recovered.* 



* It would carry me too far to dwell on the causes of infertility in critical 

 periods, but it may be well here to recall Brooks' idea that, in the female, 

 heredity and fixedness, and in the male, the tendency to variation, predomi- 

 nates. Therefore the female is the conservative, and the male the progressive 

 element in evolution. Now in many lower animals abundant food and pros- 

 perous times tend to excess of females and therefore abundance of individuals, 

 while hard times tends to excess of males and therefore to fewness of offspring 

 but to rapid variations of forms. 



