32 Arber — Atavism and Law Irreversibility. 



silence in this connection. And yet it would seem as log- 

 ical to treat them as a throw-back to some ancestor with 

 supernumerary digits, as to suppose the same thing on 

 precisely corresponding evidence in the case of a six- 

 stamened Iris. And as Bateson 15 long ago pointed out — 

 when dealing with just those types of numerical variation 

 which have been claimed as exceptions to the Law of 

 Irreversibility — a number of forms may occur through 

 discontinuous variation, which, though equally perfect, 

 cannot all be ancestral. Twenty-five years ago he wrote, 

 "In the case of Veronica and Linaria, for example, a host 

 of symmetrical forms of the floral organs may be seen 

 occurring suddenly as sports, and of these, though any 

 one may conceivably have been ancestral, the same can- 

 not be supposed of all, for their forms are mutually exclu- 

 sive. ' ' 



There is no doubt that the hypothesis of reversion has 

 too often been employed by morphologists in an uncrit- 

 ical spirit. To the students of variation and heredity 

 we owe such lucidity and precision as the term has now 

 gained, but yet biologists in general continue to use it 

 as though it retained the nebulous quality which charac- 

 terised it in Darwin's day. The only instances of genu- 

 ine atavism 16 of which we have any knowledge are those 

 which consist in the synthesis by hybridisation of some 

 original form which has now become split into different 

 races by loss of factors. But this is a totally different 

 thing from the sudden appearance of so-called 'rever- 

 sions' in cases where there has been no hybridisation. 

 The desire to interpret phenomena on the reversion 

 hypothesis may perhaps be traced to our natural mental 

 craving for similitudes, which seems easily to lead to a 

 failure to discriminate between analogy and identity. A 

 new form may recall some ancestor, near or remote, but,* 

 if the Law of Irreversibility holds, it cannot be described 

 as re-incarnating the qualities of that ancestor, except in 

 the loose and metaphorical sense in which senility is 

 described as i second childhood.' It is probably not 

 going too far to say that there is no such thing as the 

 retracing of steps, either in the life of the individual or 

 of the species: in the words of the old proverb, "The 

 baked bread can never go back to the dough." 



Balfour Laboratory, 

 Cambridge, England. 



15 Bateson, W., Materials for the Study of Variation, p. 76, 1894. 



16 The view that teratology reveals no undoubted case of reversion, is 

 maintained by Demoor, J. Massart, J. and Vandervelde, E., Evolution by 

 Atrophv, Int." Sci. Ser. vol. 87, 1899. 



