Before we can decide whether today's plants and animals are descendants 

 of ancient species, we must first answer the question Is it possible for any 

 plants or animals to have o^spring that are sufficiently different to maf^e up a 

 new species? 



It was many years after the death of Darwin before students of heredity 

 had accumulated enough knowledge to answer this question helpfully. 



In Brief 



Fossils furnish our most direct evidence about early forms of life. 



The fossil record includes entire organisms, skeletons, shells, petrifactions, 

 casts and molds. 



Similarities in the structure and life histories of living organisms suggest 

 relatedness; the greater the similarities, the less remote we assume the com- 

 mon ancestry to have been. 



Corresponding parts of different organisms built on substantially the same 

 plan are said to be homologous. The presence of homologous parts is taken to 

 indicate relatedness. 



Unlike parts of different organisms which carry on similar functions are 

 said to be analogous. 



Similarities found between the early stages of development in different 

 species are taken to indicate relatedness. 



The presence of certain useless structures within living things is difficult 

 to explain unless we assume that all plants and animals are related through 

 common ancestry. 



The uniqueness of life in isolated regions, as contrasted with similarities of 

 life in adjoining regions, can be most satisfactorily explained by assuming that 

 existing species are derived from ancient forms. 



The accumulating evidence that plant and animal species populating the 

 earth have changed in time brought two different types of answers: (1) Special 

 creation must have taken place again and again, or new species arose spon- 

 taneously again and again. (2) Life has been continuous, but species have 

 become different from their ancestors. 



The origin of species cannot be known directly. Either we depend for the 

 answer upon acceptable authority, or we build up the most reasonable hypothe- 

 sis from actual facts. 



Both the theory of natural selection and the theory of transmission of 

 acquired traits are intended to explain (1) the procession of changing plants 

 and animals and (2) the fact that each species is so remarkably well adapted to 

 live in its own special surroundings. 



469 



