CH. VI] ADAPTATION 55 



For such a process to be successful, there are other assump- 

 tions that we must make. We have (1) to assume — which goes 

 against much or most of the evidence — that a morphological 

 change has some adaptational value, (2) that such a variation 

 will appear at the time when it is wanted (for otherwise there will 

 be nothing for natural selection to work upon), (3) that the 

 conditions will continue to vary in the same direction long enough 

 to permit of the adding up of small variations until the specific 

 (sterility) line is passed, (4) that the operation is so strenuous 

 that at some point upon the way the sterility line will be safely 

 passed, (5) that at some point when the species is fully embarked 

 upon the change, a better variation, but working in another 

 direction, is not offered to it by nature, thus confusing the result, 

 (6) that when one variation has achieved its full result, it shall be 

 followed by another, often in a completely different direction 

 (for one species usually differs from another in several characters) 

 without interfering with the mutual sterility, and (7) that the 

 variation is so eminently desirable that it will be followed up 

 until the new structural feature, for instance alternate (or oppo- 

 site) leaves, palmate, pinnate, peltate, stipulate or exstipulate, 

 gland dotted, or other type of leaf, anther opening by slits, valves, 

 or pores, dorsal or ventral raphe, achene, follicle, pod, nut, 

 schizocarp, berry, drupe, etc., is fully perfected. 



The whole thing is largely based upon the third assumption 

 given above. For example, the climate (not the weather) must 

 change gradually in the direction of warmer or cooler, wetter or 

 drier. But these changes are well known to be so slow that they 

 can only be detected in averages of a century or more — a period 

 longer than the life of most plants, except many trees — whilst 

 weather is continually changeable. Suppose a plant to have 

 begun to vary in the direction of suitability to increased drought, 

 and then there comes, as so commonly happens, a cycle of wetter 

 years; what is going to happen then? Botanists have somewhat 

 neglected weather effects, when compared with agriculturists. In 

 the RepoH of the Sudan Agricultural Research Service for 1937, 

 which I have lately reviewed, it is stated that the average good 

 yields of the whole Gezira, in which the weather conditions were 

 as stated, were reflected on the Government Farm, where the 

 yields were much the same; "and once again we get an illustra- 

 tion of the comparatively small effects which local conditions 

 may have". This is familiar to all who have to do with crops, and 

 puts considerable difficulty in the way of anyone who imagines 



