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doctrine of 'evolutionary lag'. Lest we should do an injustice to her theory we must put the crucial part of her statement in her own words:-
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"In discussions on the tree and herb habit in Angiosperms, a mode of statement is often used, which seems to me to be the root of much morphological evil: I mean that in comparing various groups of flowering plants, botanists are apt to speak of some as more ancient than others. Every one of the various lineages must have lived through all the time that has elapsed since the original stock came into being, and it is hence a misuse of language to talk of one lineage as being ‘older, than another. This usage is sometimes defended on the ground that 'old' or 'ancient' in this sense means 'having primitive characteristics' but I do not think that the word 'old' should be treated as a synonym for 'old-fashioned'. Now there is a time-relation, to which attention has been drawn in Sinnott's paper on "Comparative Rapidity of Evolution in Various Plant Types," which is of the first importance when considered in connection with the equality of evolutionary age of all existing stocks of flowering plants. This point is the shortness of generations in herbs as compared with trees. Sinnott estimates that in herbaceous species we may reckon fifty to a hundred generations to a century, and in most trees only four or five. It is possible that he exaggerates this difference in general, but in some of the Gramineae it is undoubtedly even more striking than his figures suggest.
The dictum, that "the single step in evolution is not a year but a generation," must be accepted, if we believe that evolutionary development depends on the changes in the germ cells which come about in connection with sexual reproduction. It is thus obvious, a priori, that, in a given period of years, the herb will have more numerous chances than the tree of undergoing the changes in its genic outfit which condition variations. This theoretical conclusion is confirmed by the statistical facts concerning the number of species in genera and families of trees and herbs; for it is clear that the existence of a relatively high number of species in a genus or family means that it has been the scene of correspondingly active evolutionary development. Sinnott shows that in the Dicotyledons the average number of species in the woody genera is 12.5, while in the herbaceous genera it is The existence of this evolutionary lag-if we may 15. so name it-among trees as compared with herbs is well demonstrated by Sinnott, and yet he hardly seems to appreciate its full significance. In the Leguminosae, two of the subfamilies, Mimoseae and Caesalpineae, with their regular or nearly regular flowers, are without question more primitive in type than the Papilionatae, with their curious butterfly-shaped blossoms. Now of the 121 genera in the first two subfamilies, 113 are wholly woody, and the other 8 contain both woody and herbaceous forms; there are no entirely herbaceous genera. In the Papilionatae, on the other hand, 29% of the genera are herbaceous. It is evident, therefore, that the more primitive members of the Leguminosae are almost all woody, but that there is a much higher percentage of herbs in the part of the family with more specialised
July 1935,
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