76 WILLIAM BATBSON. 



of organs as in the case of the arms of Asteroidea, and 

 may be of specific occurrence as in Asterias rubens and 

 Brisinga coronata, or even ontogenetic as in Clavatella, &c. 



All the instances of repetition of organs which have been 

 so far selected, whether in the case of animals with a marked 

 long axis or in the radiate forms, have been examples of the 

 recurrence of parts or organs in some more or less definite 

 relation to the axis of symmetry of the animals. These have 

 been chosen especially as more markedly illustrating the 

 possibility that the segmentation of some forms at all events 

 may have been derived from the continual recurrence of this 

 phenomenon until it became more or less regular and trans- 

 missible to the offspring as the definite course of development. 

 But it must be remembered that repetitions of this kind are 

 of an extreme type. The recurrence of whole sets of organs, 

 as in the case of the arms of Asterias or the gastric pouches 

 and generative organs of the Nemertines, must be regarded as 

 the higher manifestations of this phenomenon, and conse- 

 quently of more or less occasional occurrence. Since, how- 

 ever, it is in these cases that the nearest approach has been 

 made to metameric segmentation as we now see it, they have 

 necessarily been selected as of the first importance. But if 

 repetitions of this magnitude are of rare occurrence, repetitions 

 of smaller parts or organs are extremely common, if not uni- 

 versal. There is hardly one of the larger or more organised 

 types in which whole tracts of the body are not composed of 

 almost precisely similar and "serially homologous" parts, 

 which are of very variable number. The scales and fin-rays 

 of fishes, the tufts of hair and markings on many caterpillars, 

 the teeth of Vertebrata, the joints of the Arthropod appen- 

 dages, or of the stems of a Crinoid, the ossifications in the 

 ambulacra of the Echinodermata, and many others, suggest 

 themselves at once. 



Especially noticeable are the casual repetition of large com- 

 plex structures, such as the mammary glands andof exoskeletal 

 organs, as the horns and dermal scutes of Vertebrates. The 



