VIII.] HOMOLOGIES. 177 



sent the most obvious and striking serial homology 

 almost the only serial homology noticeable externally. 



The facts of serial homology seem hardly to have excited 

 the amount of interest they certainly merit. 



Very many writers, indeed, have occupied themselves 

 with investigations and speculations as to what portions 

 of the leg and foot answer to what parts of the arm and 

 hand, a question which has only recently received a more 

 or less satisfactory solution through the successive con- 

 cordant efforts of Prof. Humphry, 3 Pro. Huxley, 4 the 

 author of this work, 5 and Pro Flower. 8 Very few writers, 

 however, have devoted much time or thought to the 

 question of serial homology in general. Mr. Herbert 

 Spencer, indeed, in his very interesting " First Principles 

 of Biology," has given forth ideas on this subject which 

 are well worthy careful perusal and consideration, and 

 some of which apply also to the other kinds of homology 

 mentioned above. He would explain the serial homologies 

 of such creatures as the lobster- and centipede thus: Ani- 

 mals of a very low grade propagate themselves by sponta- 

 neous fission. If certain creatures found benefit from this 

 process of division remaining incomplete, such creatures 

 (on the theory of" " Natural Selection " ) would transmit 

 their selected tendency to such incomplete division to their 

 posterity. In this way, it is conceivable that animals 

 might arise in the form of long chains of similar segments, 

 each of which chains would consist of a number of imper- 

 fectly separated individuals, and be equivalent to a series 

 of separate individuals belonging to kinds in which the 

 fission was complete. In other words, Mr. Spencer would 

 explain it as the coalescence of organisms of a lower 



3 Treatise on the Human Skeleton, 1858. 



4 Hunterian Lectures for 1864. 



5 Linnaean Transactions, voL xxv. p. 395, 1866. 



6 Hunterian Lectures for 1870, and Journal of Anat. for May, 1870. 



