36 



EVOLUTION AND ANIMAL LIFE 



they arise; others, on the contrary, long, important, and widely 

 divergent. 



The traditional tree which is drawn to explain animal 

 classification illustrates at the same time the two fundamental 

 facts upon which this classification is 

 based, namely, differentiation of struc- 

 ture, and corresponding divergence of 

 descent. All the branches of this gene- 

 alogical tree lead back, as they do in a 

 real tree, to its trunk, and the trunk 

 of this tree springs from the simplest of 

 the many-celled animals, namely, from 

 those primitive forms which resemble in 

 essential characters animals like the 



FIG. 25. Longitudinal section through the body 

 of a sea anemone: oe., oesophagus; m.f., mesen- 

 terial filaments; r., reproductive organs. 



FIG. 26. One of the sim- 

 plest sponges, Calcoh/n- 

 thus primigenius. A part 

 of the outer wall is cut 

 away to show the inside. 

 (After Haeckel.) 



simpler polyps. Indeed it seems certain that this tree trunk 

 can be traced farther back; that it must spring in the begin- 

 ning from forms essentially like the lowest organisms that we 

 know to-day, namely, single, simple cells living independently. 

 From the Amoaba to Man; that is the history of descent, or 

 ascent if one prefers. The course has been a continuous one, 

 both in point of time and in point of gradual transformation. 



