C— GEOLOGY 8i 



the condition of preservation of the specimens in the actual deposits 

 proves that the individuals v^rhose remains have been found were practically 

 contemporaneous with one another. 



In conclusion it may be said that, though the case for such an explosion 

 of serial mutants cannot be regarded as established, there is sufficient 

 evidence to warrant us in taking the suggestion seriously. Should its 

 occurrence be established it would provide a marked contrast to the 

 type of mutation made familiar by experimental work. The contrast 

 should probably be regarded as due to differences in method of study 

 and of material. The experimenter breeds with isolated and controlled 

 pairs, whilst nature breeds in a large freely mixing population with pairs 

 drawn together by instincts which for the time being are beyond the 

 experimenter's ken and across which his methods may be cutting. Made 

 matches do not necessarily yield the same results as love matches. 



The Inter-relationship of Processes. 



While, for the sake of clearness, the several processes concerned in the 

 survival during development of old characters and in the arrival of new 

 ones have been considered separately, this is not the mode of their occur- 

 rence in Nature. Here the processes may manifest themselves side by 

 side or in sequence in the same series of organisms or different processes 

 may be dominant in closely allied forms. 



Inasmuch as the particular individual that is being studied is the last 

 of an almost infinitely long series of individuals each of which started life 

 as a single cell, it seems inevitable that there should be some similarity 

 between them in the succession of stages passed through in development 

 and attained in evolution. This similarity is proportional to the proximity 

 of the ancestor to the individual that is being studied. The facts put 

 forward in the earlier pages lend strong support to this point of view and 

 emphasise the importance of recognising this similarity, which is indeed 

 the basis of all theories of recapitulation, as the background of all the 

 other processes we have been considering. These processes do but render 

 some portion more hazy and others they hide from view. 



On to this background of survivals from the past are superposed all 

 new characters. These, generally speaking, belong to one or other of 

 two categories, viz. : 



(a) Unit characters or biocharacters — features which appear fully ex- 

 pressed from the outset and undergo no subsequent change, e.g. 

 torsion in gastropods, areas of attachment in lamellibranchs. 



(6) Trend characters or bioseries — features which at the time of appear- 

 ance are almost imperceptible but which in subsequent development 

 and evolution become progressively more fully expressed, e.g. 

 length of septa in corals, coiling in Gryphcsa. 



Unit characters may appear coenogenetically, that is to say, at some 

 early stage in development. Their appearance may open the way to 

 other changes of a serial quality. Thus, for example, the twisting of the 

 visceral hump in gastropods was followed by the progressive reduction. 



