12 CONTACTS WITH DARWINISM I [ch. ii 



characters must be correlations. And why did one not find, in the 

 fossil records, any species that had been fossilised before this 

 complicated process had been completed? 



It is clear that, on the theory of gradual adaptation, a very 

 long time must be allowed to get from one species to another. 

 This means that the change of conditions must go on for a long 

 time also, for if a small change in structure enabled the species 

 growing in one locality to survive there, there would be no urgent 

 reason why they should continue to vary in the same direction, 

 unless the conditions also continued to vary in the same direction 

 as that in which they had begun to do so. 



Another difficulty was to understand why variations of this 

 kind should usually go so far as to pass what one may call the 

 rough-and-ready line of distinction between species — that they 

 should be, mutuallv, more or less sterile. 



One does not find to anv serious extent in the fossil record, 

 species which represent real intermediates between existing or 

 fossil species. One finds rather examples of species that have 

 some of the characters of one, some of another. But one does not 

 find species (as from the constant occurrence of the few characters 

 side by side in existing species one might expect to do) that show 

 intermediate characters between alternate and opposite leaves, 

 between palmate and pinnate leaves, between erect and climbing 

 stems, between racemose and cymose inflorescences, between 

 flowers with and without a cyclic perianth, between isomerous 

 and heteromerous flowers, between imbricate, valvate, and con- 

 volute aestivation, between flowers with the odd sepal posterior 

 and with it anterior, between stamens in one and in more whorls, 

 between anthers opening by splitting or by teeth, valves, or pores, 

 between 3-locular and 4-locular ovarv, between ventral and dorsal 

 raphe, between loculicidal and septicidal fruits, and so on through 

 all the important structural characters. 



All these were very serious difficulties, while it had also to be 

 remembered that in any case evolution could only go on if the 

 needful variations in the right direction should appear, for, unless 

 this should happen, it was evident that natural selection could 

 do nothing. One could not imagine the "mixed" variation of 

 characters a, &, c, etc., above-mentioned appearing at all, unless 

 most of it was simply correlation, and if the differences had to 

 appear one by one, the chance of all appearing was but small, and 

 the time required would be enormous. Forty years ago it was 

 clear to the writer that some form of sudden and irreversible 



