122 BIOLOGY: GENERAL AND MEDICAL 



Among the porifera or sponges, as among the higher 

 ccelenterates, the mesenchyme increases in importance 

 so that we have three tissues, the ectoderm, the entoderm, 

 and the mesenchyme, which may be regarded as the start- 

 ing point of all the future differentiations for even among 

 the highest animals, and in man himself, the tissues are 

 still divided into those springing from one or the other 

 of these three structures, the outer of which is the source 

 of the integuments, the inner the source of the organs 

 of digestion, and the middle the source of the organs of 

 support and locomotion. 



The increasing structural complexity appears to be 

 largely a matter of necessity. As the number of cells 

 increases, and specialization of these cells is gradually 

 effected, it becomes inevitable that all should not be 

 favorably situated with reference to the source of nu- 

 trition, so that some means must be provided for con- 

 veying suitable pabulum to them. As more of the 

 nutritive pabulum is required, greater perfection of the 

 organs of motion or prehension must be developed in 

 order that it may be supplied, and greater perfection 

 of the digestive organs becomes necessary that none of 

 it wastes. As the differentiation of parts becomes more 

 and more perfect, and their interdependence increases, 

 means by which one part may communicate with another 

 appears in the form of the nervous system. 



Should we, however, seek to explain complexity of 

 structure by conditions intrinsic in the organism itself, 

 and solely along the lines suggested in the preceding 

 paragraphs, we must inevitably fail. Such conditions 

 would determine a uniform evolution of the developing 

 substance and not result in the diversity of structure 

 found among the many phyla of plants and animals. 

 The chief sources of structural modification lie outside 

 of the organism in its environment as will be explained 

 in a subsequent chapter. 



We will now endeavor to trace each of the important 

 systems of organs back to its inception, remembering 



