Many instances, noted in our former lectures, could be quoted here. The systematic distribution of rayed and rayless species and varieties among the daisy-group of the composites affords a long series of examples. Accidental variations in both directions occur. The Canada fleabane or _Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some others may at times be seen with ray-florets, and according to Murr, they may sometimes be wanting in _Aster Tripolium_, _Bellis perennis_, some species of _Anthemis_, _Arnica montana_ and in a number [237] of other well-known rayed species. Another instance may be quoted; it has been pointed out by Grant Allen, and refers to the dead-nettle or Lamium album. Systematically placed in a genus with red-flowering species, we may regard its white color as due to the latency of the general red pigment. But if the flower of this plant is carefully examined, it will be found in most cases not to be purely white, but to have some dusky lines and markings on its lower lip. Similar devices are observed on the lip of the allied _Lamium maculatum_, and in a less degree on the somewhat distant _Lamium purpureum_. With _Lamium maculatum_ or spotted dead-nettle, the affinity is so close that even Bentham united the two in a single species, considering the ordinary dead-nettle only as a variety of the dappled purple type. For the support of this conception of a specific or varietal retrograde change many other facts are afforded by the distribution of the characteristic color and of the several patterns of the lips of other labiates, and our general understanding of the relationships of the species and genera in this family may in a broad sense be based on the comparison of these seemingly subordinate characteristics. The same holds good in many other cases, and systematists have often become uncertain [238] as to the true value of some form, by its relationship to the allied types in the way of retrogressive modification. Color-differences are so showy, that they easily overshadow other characters. The white and the blue thorn-apple, the white and the red campion (_Lychnis vespertina_ and _diurna_) and many other illustrative cases could be given, in which two forms are specifically separated by some authors, but combined by others on the ground of the retrograde nature of some differentiating mark.