THE INDIVIDUAL AND THE SPECIES 163 



there is differentiation of tissues in the multicellular 

 organism. 



Again, the Ccelenterates provide examples of 

 animals which are aggregates of parts, each of which 

 is the morphological equivalent of a single Hydra. 

 Such an animal as a Siphonophore, for instance, 

 consists of zooids, and each of these units has the 

 essential structure of a Hydra. But the zooids are 

 not all alike : some of them subserve the function of 

 locomotion, others of aggression, others of digestion 

 and assimilation, and so on. Here, again, the whole 

 organism may be mutilated ; parts may be removed 

 and regeneration may occur ; but, as a Siphonophore, 

 all of the different zooids must be present if the char- 

 acteristic functioning of the animal is to continue. 



The Protozoon is, therefore, an individual of the 

 first order, the Hydra an individual of the second 

 order, and the Siphonophore an individual of the 

 third order. Some such conception of degrees of 

 individuality will probably be regarded as satisfactory 

 by most zoologists, yet consideration will show that 

 it is very inadequate. Many unicellular plants and 

 animals may consist of a number of cells, which are 

 all alike. The Diatoms and Peridinians reproduce by 

 the division of their cell bodies and nuclei, and the 

 parts thus formed may remain in connection with each 

 other. A Diatom may consist of one cell, or it may 

 consist of a variable number of such connected together 

 by filaments, or in other ways ; and the dissociation 

 of such a series may occur without interfering in any 

 way with the functioning of the parts separated. A 

 Tapeworm consists of a " head " or scolex, containing 

 a central nervous mass and organs of fixation ; and 

 organically continuous with this is a series of segments 

 or proglottides. These proglottides are formed con- 



