CHAPTER 25 • HOW SPECIES HAVE ARISEN 



1 What causes new species to arise? 



2 How do new species come to fit their surroundings? 



3 Are modern plants and animals superior to ancient forms? 



4 How can we tell whether any kind of plant or animal is really a 



new species? 



5 What kinds of variations are inherited? 



6 Why are some variations more fit than others? 



7 Does the human race consist of one or of several species? 



8 How can we tell whether man has resulted from evolution? 



9 What is meant by a "missing Hnk"? 

 10 Is evolution taking place today? 



All life is one. Every plant is like all other plants, every animal is like 

 every other — in the basic capacities. That is, each grows, develops, responds 

 adaptively to what goes on around it, reproduces. 



Yet every individual is unique. Indeed, the individual is all that we can 

 know directly — the individual, and many other individuals more or less Hke it. 

 From our experience with many unique individuals we may feel that we know 

 whole classes of similar individuals. We speak with confidence of the cat or 

 dog family, of the class "fishes", of the order "beetles", or of all mankind. 



Since individuals resemble their parents and other ancestors, they form 

 groups that remain fairly constant through many generations. But individ- 

 uals also differ from their parents, as well as from each other. The actual 

 constitution of a species or of a genus is constantly changing, just as the exact 

 chemical make-up of an individual is constantly changing. But does this 

 process bring about the formation of new species? And how, in spite of such 

 changes, do living things continue to be adjusted to their surroundings? 



How Can New Species Arise Out of Old Ones? 



New Species or New Individuals There can be no doubt that species 

 of plants and animals became extinct throughout ancient times, and that new 

 species came into being from time to time. How can a new species arise 

 ready-made, with a complete set of individuals at all stages of development, 

 like the inhabitants of a beehive? But it is no easier to imagine a species 

 starting out as a pair of adults, or as a number of eggs, which would first have 

 to develop into adults and then reproduce themselves. Cuvier cut across all 

 such difficulties by saying simply that when the time came, new species were 

 created, and they repopulated the world. And the new species, he was sure, 

 had no connection whatever with their predecessors, although they had been 

 created along similar lines. 



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