544, WILLIAM BATESON. 
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 and of exoskeletal 
organs, as the horns and dermal scutes of Vertebrates. The 
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