238 G. Udiiy Yule. 
procedure he will attain any desired end with greater certainty and 
speed. As regards the latter, the fact that heredity may he vanishingly 
small within the pure line, although quite sensible within the 
population at large is a very striking fact and, I believe, a new one. 
It must find its proper place and explanation in any complete 
theory of intraracial heredity. 
Professor Johannsen’s explanation, as I understand it, is this, 
that the race consists of a number of distinct “ types,” in each of 
which the germ-plasm st»ucture (or whatever we choose to term 
the character of the germ cell that determines the character of 
the soma) is unalterable ; that in a pure line all the variations are 
consequently purely somatic and therefore non-heritable; and 
that selection cannot operate in such a race because there are no 
germinal variations to afford the necessary material. On this 
hypothesis the heredity within the pure race is absolutely zero. 
The hypothesis seems, however, to be somewhat qualified by 
later passages. The meaning of the word “Typus” 1 is not defined. 
I have assumed that the writer means an unalterable germinal 
structure, as it is only on such an assumption that the conclusions 
seem to follow. But on pp 61-62 of the summary, after a 
repetition of the statement to the effect that it is the “ type of the 
line ” in conjunction with the environment which determines the 
character of the offspring, and an approving quotation of De 
Vries’s dictum that the line is vollig konstant und hochst variabel ” 
(germinally constant, I take it, and somatically variable) we read 
“ Damit sei aber durchaus nicht gesagt, dass die reinen Linien 
absolut constant sein sollen.” 2 In the first place, it is suggested, 
1 Statistically, the word is used 111 two quite distinct senses, 
neither carrying any implication of stability in the bio¬ 
logical sense. (1) One may speak of the type of the race as 
regards any given character, meaning thereby the modal or 
most frequent value of the character (identical with the 
average, if, but only if, the frequency distribution be 
symmetrical). (2) One may speak of organisms "of a given 
type” within the race, lueaniug thereby simply organisms 
possessing a given value of the assigned character, within 
more or less narrow limits. 
2 I met with precisely the same difficulty in reading, some 
months ago, the very interesting work of Klebs on 
“ Willkiirliche Entwickeluugsandenuigen bei Pflanzen.” 
O11 p. 145 is stated “ Da wir von del* Voraussetzung ausgeheii, 
dass jeder Spezies eine konstante spezifisclie Struktur zu 
Grunde liegt.” while three pages later we find 
“ Die Anfgabe der Speziesphysiologie ist aber nocli aus 
einem zweiteu Grunde grenzenlos weil die Konstanz der 
spezifischen Struktur.dock nur relativ ist.’’ The 
fact however that it is “relative” (ie . really not constant 
at all) would have been better stated earlier. It limits 
many of the author’s conclusions to an extent he does 
not realise. 
