C— GEOLOGY 71 



its direction was predetermined at the outset, or whether it was induced 

 and selected by circumstances at an early stage, we cannot tell ; but when 

 once it has been fairly started, it continues inexorably to its limit. It 

 must be admitted that this view of evolution is out of favour with many 

 neontologists, . to whom the word ' orthogenesis ' is anathema. The 

 attitude of these critics has a precedent in that of the physicists of a past 

 generation who were convinced that the sun could not have existed long 

 enough for geological history to have happened. Inability to explain a 

 fact is no evidence of its fallacy ; and palaeontologists can proceed un- 

 ruffled to record the facts of orthogenesis. 



Whatever may be the influence, direct or indirect, of environmental 

 changes on the course of evolution, there is certainly this other factor at 

 work. The several organs of an organism have considerable independence, 

 although they must keep a harmonious balance if disaster is to be averted. 

 In the nature of things, palasontological evidence is most adequate for 

 appreciation of the evolution of such structures as shells. These external 

 organs are at once intimately concerned with the environment, and capable 

 of much modification without affecting the welfare of the organism of which 

 they are a part. The dual nature of morphological evolution in cases of 

 this kind is very clearly shown in the Ostrea-Gryphcea lineage worked 

 out by Trueman. 



If we consider a flat oyster-shell, such as Ostrea liassica, affixed by the 

 greater part of one valve, two obvious imperfections appear. Only a 

 limited number of individuals can occupy a definite space if they have this 

 posture ; and the valves will open near ' ground-level ' where the water 

 may be gritty. Whether by some intrinsic impulse, or by selection of 

 chance variations, such a type of oyster tends, in the course of many 

 generations, to reduce the area of fixation, and, by curving the released 

 part of the valve upwards, to reach purer and less crowded water. In 

 course of time this tendency is pushed back into earlier stages of develop- 

 ment, until a type appears which is fastened by a very small area only, 

 and in which the direction of growth has been rotated through 90 degrees. 

 In this state the oyster has rectified both of the disadvantages inherent 

 in its first condition, for now many shells can stand erect where previously 

 one lay prone, and all open into the water above the mud-level. 



So far in the story (which is demonstrably true) it is possible to invoke 

 the influence of environment as a causative and selective influence. But 

 the story does not end here. Gradually, in direct continuation of what 

 had happened hitherto, the area of fixation becomes more and more 

 reduced and the curvature of the shell more pronounced, until once again 

 the opening of the valves is bent downwards towards the sea-floor, and the 

 area occupied by the individual is again large. Still, as we follow the 

 sequence, the rotation continues, until the final spot of fixation is 

 obliterated, and the shell, with one valve curved through a semicircle, 

 lies loose. Freed from the restraint 6f fixation, this valve becomes pro- 

 gressively more enrolled, until it assumes the familiar spiral form seen in 

 Gryphcea incurva. As a compensation for freedom, and the inconvenience 

 and danger thereby caused, the enrolled part of the valve becomes enor- 

 mously thickened, so as to ensure uprightness of posture by the same 



