464 University of California Publications in Zoology [Vol. 18 



plate or pair of plates, or involving a sudden rearrangement of the 

 plates of one or another of the four variable patterns ; and that each 

 pattern with its own mode of variation for the length of its critical 

 suture marks a settled arrangement of the plates in which they are 

 for the time being at least in equilibrium. 



24. To judge from the constancy of a given pattern in a given 

 well recognized species, it seems to be usual that an influence suffi- 

 cient to upset the plate arrangement in one or more of the variable 

 regions is of sufficient moment to cause other changes also, the sum 

 total of which would place the organism in quite a new species. That 

 this is not always the case appears from a few exceptional occurrences 

 of a change in the ventral plate pattern, without any other morph- 

 ologic changes which would warrant placing these individuals in 

 another species. 



25. These exceptional cases of unusual changes in the ventral plate 

 pattern of the species just mentioned are to be regarded as reversions 

 from a specialized to a more generalized type ; that is, these changes 

 may be due to either an inhibitory factor suppressing the full develop- 

 ment of the ventral plates usual for the species, or more probably to a 

 lack of the usual stimulus for growth in plates V and 7", on account 

 of which a more primitive plate pattern is produced. Any interference 

 with the normal progress of a race of these organisms seems to result 

 in an inhibition or retardation of progress and fixation at some phylo- 

 gentically earlier stage of development, rather than in the stimulation 

 of new lines of variation. The normal progress of development in this 

 family is suggestive in certain respects of an orthogenetic mode of 

 evolution. 



26. These changes in plate pattern within a species or between 

 species are, therefore, to be regarded as mutations, and their signifi 

 cance is very great in suggesting the mutationary method of species 

 formation in this group. 



27. The suture or plate pattern is, of course, only a superficial 

 indication of a deeper seated variable factor governing the orderly 

 skeletal fragmentation and plate growth characteristic of this family. 

 The kaleidoscopic readjustment of the plates may be of secondary 

 importance and may be induced by some physical quality of the 

 skeletal material which retains its inertia until acted upon by a suffi- 

 cient accumulation of strain to cast it over into another state of tem- 

 porary equilibrium. Such a circumstance would, however, even if the 

 underlying form were more regular in its action, alter the mutationary 



