CHAPTER III 

 MULTICELLULAR ORGANIZATION: PHYLOGENETIC 



ACCORDING to the doctrine of descent, which at present receives wide 

 recognition, all animals have evolved from some simpler forms. It is 

 held that every multicellular animal or plant has developed through an 

 infinite series of stages from a unicellular form. The geological forma- 

 tions bear broken records of such an advance in structure. This past 

 history of a race is called its phylogeny. Necessarily it is a history of 

 which we have no human records, since man did not exist during the 

 longer early periods and was not a scientific investigator until a com- 

 paratively recent time. Our only actual evidence lies in comparative 

 study of the fossil remains of such creatures as happened to be preserved 

 in the rocks. Of these we have only the hard parts, as bone and shell 

 in most cases, and but few actual histological structures have been pre- 

 served, as in the case of some selachian muscle and some crustacean 

 integument. 



These fossil remains show that in earliest times only the simplest 

 forms existed, and that, as ages passed, larger as well as more complex 

 forms were added. Some races were entirely lost during these changes. 

 The geological record also shows that the simplest and intermediate 

 groups continued to exist with little change to the present time, except 

 some that have been lost entirely. Therefore, we may assume that the 

 present large series of animals, known as the taxonomic series, represents, 

 to a certain degree, the long extinct phylogenetic series whose broken 

 record we find in the rocks. 



The taxonomic series suggests that higher efficiency in organization 

 has been effected along two lines : First, by an increase in the mass of 

 the individual; and secondly, by a differentiation of the component cells 

 of an individual. How these modifications were accomplished can, to a 

 certain degree, be .understood by a study of certain living forms. 



We have seen in the preceding chapter that, though the specialization 

 in cellular organization be varied, the maximum size of cells is soon 

 reached. It is only exceptionally that unicellular creatures attain 

 macroscopic dimensions. Increase of bulk is, therefore, rarely effected 

 by the growth of a single cell and is usually accomplished by the 



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