DISCONTINUOUS VARIATION. 55 



recently obtained by Herbst.* He found in no less 

 than ten different species of Crustacea, belonging to 

 four different families, that if an eye stalk were totally 

 extirpated, there always grew up in its place a hetero- 

 morphic new structure, like an antennula, which bore 

 olfactory hairs. If, however, only the eye were re- 

 moved, and the stalk together with the ganglion left, 

 instead of the antennula there arose the beginnings of 

 a new eye. A similar result to this was obtained on ex- 

 tirpation of either stalk or eye in Porcellana platycheles, 

 presumably because in this species the stalk contains no 

 ganglion. 



The chief contents of Bateson's book may be very 

 briefly summarised, in order that the reader may gather 

 some idea as to the kind of evidence on which Bateson 

 founds his argument. A considerable body of evidence 

 is given concerning variations in the numbers of ver- 

 tebrae and ribs, the most important conclusions being 

 that the variations are considerable, especially in some 

 types such as Simla satyrus, the Bradypodidse, and 

 Bombinator igneus, and that imperfect vertebrae are 

 very rare. Turning to Invertebrates, it is shown that 

 among Oligochaeta and Hirudinea, certain forms, e. g., 

 Perionyx excavatus and Pachydrilus sphagnetorum, 

 have great variability, whilst others, such as the com- 

 mon earthworm, rarely vary. Both forward and back- 

 ward Homoeosis may occur, forms which normally have 

 the male pores on the 15th segment having them on the 

 16th, or on the 13th. Returning to Vertebrates, evi- 

 dence is next adduced concerning cervical fistulae and 

 auricles, and supernumerary mammae. Variations in 

 * Arch. f. Entwickelimgsmechanik, Bd. ix. p. 215, 1899. 



