CHAP, i.] ARRANGEMENT OF EVIDENCE. 85 



For the word 'Metamorphy' I therefore propose to substitute tin- 

 term Homoeosis, which is also more correct ; for the essential 

 phenomenon is not that there has merely been a change, but that 

 something has been changed into the likeness of something else. 



In the cases given above, the distinction between Homoeotic 

 Variation and strictly Meristic Variation is sufficiently obvious, 

 but many numerical changes occur which cannot be referred with 

 certainty to the one class rather than to the other. Such cases 

 are for the most part seen in Vertebrates : for in them what may 

 be called the fundamental numbers of the segments are not consti- 

 tuted with the definiteness found in Arthropods or in the Annelids, 

 and several Meristic series of organs are disposed in numbers and 

 positions independent of, or at least having no obvious relation to 

 those of the other Meristic series. The number and positions of 

 mammae, or stripes, for instance, need not bear any visible relation 

 to the segmentation of the vertebrae &c. The repetition of mem- 

 bers of such a series may thus not coincide with, or occur in mul- 

 tiples of the segmentation of other parts in the same region. When 

 such is the case, when the segmentation of one series of organs 

 bears no simple or constant geometrical relation to the segmenta- 

 tion of other systems, it is not always possible to declare whether 

 a numerical change in one of the systems of organs belongs properly 

 to the first or the second of the classes described above. It is 

 likely enough that in such a case as that of mamma?, there may 

 sometimes be an actual Meristic division and subsequent separation 

 of the tissues already destined to form the mammae, occurring in 

 such a way that each comes to take up its final position, and 

 indeed the numerous cases in which such division has been 

 imperfectly effected go far to prove that this is the case. But, on 

 the other hand, it is not possible to know that the division did not 

 occur before any tissue was specially differentiated off to form 

 mammas, and that the separation may be as old even as the 

 division of the mammae of the right side from those of the left, a 

 process which almost beyond question occurs in the segmentation 

 of the ovum. The distinction between these two alternatives is 

 thus one rather of degree than of kind, and it is only in such forms 

 as the Arthropods, the floral organs of some Phanerogams and the 

 like, where the members of the several Meristic series have definite 

 numbers, or coincide with each other, that this distinction is easily 

 recognized. For this reason I do not think it well to attempt 

 to carry out any classification of the evidence based on this dis- 

 tinction. 



In the foregoing remarks I am aware that a very large question, 

 which lies at the root of all accurate study of Meristic Variation, 

 has been passed over somewhat superficially, but I scarcely think 

 a fuller treatment possible in the present state of knowledge of 

 the physics of Division, and in the absence of thorough observation 

 of the developmental history of those tissues which ultimately 



