THEORY OF EVOIiUTION 481 



sources give incomplete and sometimes inconclusive evidence of the 

 history of any particular organism. The individuals are recognized 

 as being only points on a long line of modifications. The two ends 

 of this progressive line are seldom recognized with any finality. 

 Darwin, and many others since his time, recognized three funda- 

 mental facts that bring about a progressive continuity among living 

 things : heredity, with the tendency for organisms to resemble their 

 parents; variation, in that no two organisms are exactly alike, hence 

 the resemblance between parent and offspring is not absolute; and 

 constant production of more offspring than can survive. The phylo- 

 genetic relations of animals show all indications of having been 

 brought about by the operation of the above-mentioned factors in an 

 orderly, evolutionary progress. 



Variation is one of the most obvious facts and consistent phe- 

 nomena in the animal kingdom. Darwin held that variability is 

 axiomatic among living things. As long as differences among indi- 

 viduals of a group are minor or are irregularly distributed, the 

 group is likely to be considered a species. However, the species is 

 not rigidly bounded, and it must be remembered that continuous 

 variation is in progress within this group as one generation follows 

 another. It is becoming apparent to man that new groups have 

 arisen and are arising within old groups of animals, also that old 

 groups have broken up, some to become new ones and others to be- 

 come extinct. These new groups were separated by small differ- 

 ences at first but gradually they attained greater and greater diver- 

 gence from the common form. This is particularly likely where the 

 different groups have become widely separated geographically or 

 exist under distinctly different conditions of life. Frequently, it is 

 only the widely separated extremes of the group which carry on 

 the posterity, the intermediate forms having died out. In this way 

 rather widely separated specieg may have arisen from common 

 stock, and the lack of intermediate forms may accentuate the wide 

 divergence between them. During the millions of years which life 

 has existed on earth, it seems evident that this process of divergence 

 between groups has been in operation until there now exist many 

 definable groups with distinct lines of relationship. Our natural 

 system of classification is based on the relations and differences 

 established for the different animal groups. Comparative studies of 

 numerous animal groups will help to show some of the relationships. 



