Ill USELESS CHARACTERS 75 



of special adaptation. It is known, for example, that in cal- 

 careous sponges the variation and disappearance of one axis 

 of the spicule actually forms a transition from one of the 

 principal divisions of the order into another, a subject which 

 will subsequently be discussed fully in connection with a 

 particular case. 



And of what use w^as the gradual modification of the 

 Ammonite's shell in such a way that new characters, some of 

 a very beautiful kind, arose always round the aperture, and 

 extended in the descendants more and more over the whole 

 structure, so as to form new species, as Wurtenberger has 

 shown? How can these characters, especially at their first 

 origin, have been useful ? 



And what demand upon the organism could entirely 

 exclude the development of such indifferent characters ? 

 Certain it is that this one or that amon^ such characters was 

 once useful to the ancestors — as, for example, possibly the 

 traces still occurring of marking in Canid^e-^ (dog, wolf, 

 jackal), or possibly the stripes on the shell of our garden 

 snails. Helix hortensis and nemoralis. But it is equally 

 certain that this does not hold for all. 



I mentioned the garden snails as an example intentionally. 

 It is the less easy to discern any use in the striping of the 

 shells of these snails ; not only does the striping vary greatly, 

 but it is about as often absent as present. The striping 

 might be regarded as an ornament which acted as an advan- 

 tage in sexual selection. But such an assumption is incon- 

 sistent with the following facts : I have observed for years in 

 my garden that striped and unstriped individuals of Helix 

 Iwrtensis unite without any selection. And what makes this 

 case particularly noteworthy as a support for my view of the 

 comparatively slight effect of sexual crossing in the production 

 of intermediate forms (one-sided heredity) is this, that the 



^ Cf. my articles in Ilnmholdt. 



