ORIGIN OF ORGANIC FORMS 315 



entirely regular form. This phenomenon is observed 

 in campanulas and other plants. On the other hand, 

 in plants with entirely symmetrical flowers, such as 

 those of the Labiateae, Orchidaceae, and other families, 

 the apical flower often acquires a regular form. This 

 is the case in the Sage. All the lateral flowers have a 

 characteristic two-lipped form, while the apical flower 

 is sometimes quite regular. Recent experiments have 

 adduced direct evidence in support of this suggestion.^ 

 On removing the effect of the earth's attraction by 

 means described in chapter vii., it was found possible to 

 transform bilaterally symmetrical flowers artificially into 

 regular, radially symmetrical flowers. Thus the first 

 appearance of longitudinal symmetry in a flower may be 

 ascribed to the action of the force of gravity, the rest will 

 follow as the effect of selection. Since it is beyond doubt 

 on the one hand that cross-fertilisation effected by insects 

 is useful to plants and gives rise to a more vigorous and 

 healthy generation, and on the other hand that for 

 insects, which visit the flowers for their honey, the lower 

 lip of the flower presents a convenient platform, it is 

 clear that in every generation amid plants struggling 

 for existence those which possess this bilabiate form 

 more sharply differentiated have more chance of survival. 

 Similarly, partly owing to the force of gravity and 

 partly on the strength of the law we have just mentioned 

 of correlation of growth, first one, then all the three 

 upper stamens become atrophied, while the two lower 

 ones increase in size, and under the influence of selection 

 develop their peculiar shape, which is that most useful 

 to the plant. We gather from this example that in order 

 to explain the origin of a given form, be it even a very 

 complicated one, we have but to settle the following 

 three points : that the original variation might have 

 arisen under the influence of physical forces (acting upon 

 the organism generally in its embryonic stage, and only 



1 H. Spencer was the first who advanced it. 



