864 TEXTBOOK OF ZOOLOGY 



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. As long as differences among in- 

 dividuals of a group are minor or irreguarly distributed the group 

 is apt 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 become extinct. These new 

 groups were separated by small differences at first but gradually they 

 attained greater and greater divergence 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 species 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 rela- 

 tionship. Our natural system of classification is based on the rela- 

 tions and differences established for the different animal groups. 

 Comparative studies of numerous animal groups will help to show 

 some of these relationships. 



Colony Formation in Certain Protozoa 



Due to incomplete separation of cells following division in Pro- 

 tozoa, colonies are begun. In some instances groups of cells in some 



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