INTRODUCTION 5 



It is not my intention to deal with the whole of this sub- 

 ject at present. I will merely mention one or two special 

 cases which are included in it. We may consider characters 

 diagnostic of a kinship of any degree, from the most general 

 to the most particular, from the sub-kingdom to the species 

 or even the variety. We must remember that the subject is 

 not exhausted by multiplying cases of adaptation, which are 

 numerous enough throughout the animal kingdom. We 

 must take care to include all the characters which are really 

 constant, hereditary, and peculiar, and enquire whether they 

 are all adaptive. On the present occasion I will content 

 myself with taking the most general and the most special 

 cases of kinships, namely the sub-kingdom and the species. 



The principal sub-kingdoms or phyla of animals are the 

 Ccelenterata, the Echinodermata, the Cha3topoda,the Mollusca, 

 the Arthropoda, and the Vertebrata. In the first two the 

 general plan of the body exhibits radial symmetry, in the 

 last four the body is bilaterally symmetrical. It may be 

 maintained that in animals which are free and active, and 

 move in one direction in search of food, bilateral symmetry 

 is an advantage, favouring regular and definite movements. 

 But on the other hand, why should the starfish and the sea- 

 urchin consist of five similar parts arranged regularly round 

 a central axis? These animals are free and active and yet 

 are not bilaterally symmetrical. It might be suggested that 

 they obtain their food in such a manner that it is of greater 

 advantage to them to be able to move with any part of the 

 body foremost, than with always the same part foremost. 

 But this would not explain why the generative organs are 

 also situated in radially symmetrical positions. It might be 

 urged that this was merely a consequence of the general 

 structure of the body. But this is exactly what I wish to 

 maintain. All the peculiarities are intelligible if we regard 

 them as consequences of the conditions of life and growth. 



