970 PRINCIPLES OF STRATIGRAPHY 



ceding diagram (Fig. 257) will summarize this method -and also 

 give a graphic illustration of the law of acceleration. 



Origin and Development of i haractcrs tlic Important Question: 

 Rectigradations and AUometrons. As pointed out by Osborn, the 

 origin and development of individual characters or parts is the 

 important subject for investigation, the species cjuestion being of 

 minor significance. It has been shown in the preceding sections 

 that characters develop more or less independently of each other, 

 and also that they develop in recognizable directions, or ortho- 

 genetically. Such definitely developing characters when arising as 

 new characters are termed by Osborn rectigradations; whereas, if 

 they are due to a change in proportion of such characters, they are 

 termed by him ^lllometrons (30.:32). 



Nomenclature of Stages in Development. 



Ontogenetic Stages and MorpJiic Stages. In the preceding ex- 

 amples it will be noted that the stages dealt with are form stages 

 or morphic stages only, and that they have no constant relation to 

 the actual stages in successive ontogenetic development. Thus, 

 one and the same morphic stage, i. e., stage characterized by certain 

 morphological characters, as, for example, ribs, tubercles, or spines, 

 etc., may be characteristic of the adult of one individual, and, of a 

 more youthful stage, of another. In dealing with changes in form 

 it is desirable to refer each morphic stage to the corresponding adult 

 stage of an ancestor, and to designate it by the name of that ances- 

 tor. Thus the tubercled stage of Fulgiir tritonis (Fig. 256, /) is des- 

 ignated the F. fusiformis stage, since the feature in question charac- 

 terizes the adult of that species. In like manner, the smooth 

 morphic stage of F . tritonis is designated the maximum stage, and 

 the spinose stage the F. caricum stage, from the species in which 

 these characters belong to the adult. The development of each in- 

 dividual (ontogeny) comprises a series of stages which develops 

 from birth to old age. These ontogenetic stages, or onto-stages, 

 are similar in time-duration for relaj-gd organisms and are further 

 characterized, in a general way, by ^ correspondence in the pro- 

 portional rate of growth in closely related types. They are, how- 

 ever, independent of the morphic characters, for, as already shown, 

 a certain morphic character may appear in one individual in the 

 adult stage and in another more accelerated individual in a more 

 youthful stage. (Grabau-iia.) 



Simple Organisms. Hyatt and others have given us a set of 



