76 ANIMAL INDIVIDUALITY [CH. 



but then first a part and finally a whole of one of the 

 forms comes to have a separate existence in space 

 simultaneously with one of the later forms. So, he 

 argues, since the tadpole and the frog can rightly be 

 called mere forms or phases of the same individual, 

 then the Pilidium and the Nemertine, and then all the 

 polyps in .the hydroid colony 1 , are but such forms 

 too. 



As a matter of fact, this gradation does not seem 

 to exist in nature ; but even if it did it would not be 

 convincing. It is often forgotten that the most perfect 

 quantitative gradation from one condition to another 

 is no guarantee that the two conditions shall not be 

 qualitatively different. To take the simplest example, 

 when the chemical substance denoted by the symbol 

 H 2 is heated, a definite addition to the rate of motion 

 of its molecules is made for each degree of tempera- 

 ture through which it is heated. This quantitative 

 addition, however, has a qualitative result: with 

 continued heating the substance passes from the 

 solid state into the liquid, and from that into the 

 gaseous, turning from ice to water, from water to 

 steam. There is a similar gradual transition in life 

 from the mere aggregate to the higher-grade in- 

 dividual (Chap. IV). 



Here, however, there seems to be no such series. 



1 The examples actually used by him are the Salpae and the 

 Aphides. 



