f 
- PART I. | - HISTORICAL INTRODUCTION. 21 
among homostylic forms, in which the possibility of self-fertilisa- 
tion is unchecked, those with regular flowers stand, according to 
Axell, on a higher grade that those with irregular flowers, since 
they permit the visits of more various insects. Axell thus arrives 
_ at the following order of development :— 
, 
e 
3 
‘ 
: 
‘ 
: 
, 
7 
A. Flowers which are fertilised by the aid of an extraneous 
medium of transport (jlores chasmogamt). 
I. Anemophilous forms: (a). dicecious, (b) moneecious, (c) 
dichogamic (proterogynous), (7) homogamic. 
If. Entomephilous forms : 
a. Self-fertilisation hindered by 
1, Diclinism ) Two insect-visits necessary for each 
2. Dichogamy § act of fertilisation. 
3.: Herkogamy? 4.5.05: + « Only one insect-visit 
b. Self-fertilisation not hiner necessary for each 
1. Heterostyly. 2. Homostyly act of fertilisation. 
B. Flowers which fertilise themselves without the aid of an 
extraneous medium of transport ( flores cleistogamt). 
If we review the lines of research above described, in order to 
get a clear idea of which path leads us most surely to the condi- 
tions determining the forms of flowers, it becomes plain that two 
different aims must be kept in view; these are closely connected, 
"and sometimes within certain limits mutually depend upon one 
_ another, but they cannot be confounded without danger; viz. 
(1) the elucidation of floral mechanisms, and (2) the proof of the 
Knight-Darwin law. 
Sprengel kept in view as the sole aim of his researches the 
_ explanation of the characters of flowers on certain presumptions 
_ which were to be justified by the constant possibility of such expla- 
nations; and his results were singularly fortunate so far as was 
possible with his teleological conceptions, and without knowledge 
or foreshadowing of the advantage of cross-fertilisation. Darwin 
_ remedied both flaws in Sprengel’s theory, since he not only gave 
complete expression by his theory of natural selection to the new 
conceptions that had gradually grown up, but he also made the 
definite statement, and took steps to prove it, that the effect of 
_ cross-fertilisation is advantageous. But he did not content himself 
with the statement that cross-fertilisation leads to a more vigorous 
offspring than self-fertilisation, which would have sufficed pro- 
 yisionally to explain the contrivances in flowers, but he laid down 
