are presented together, we are better enabled to trace them.
646.
Lastly, although not strictly in the same class, the iridescent appearance of certain molluscæ may be mentioned, as well as the phosphorescence which, in some marine creatures, it is said becomes iridescent just before it vanishes.
647.
We now turn our attention to those creatures which belong to light, air and dry warmth, and it is here that we first find ourselves in the living region of colours. Here, in exquisitely organised parts, the elementary colours present themselves in their greatest purity and beauty. They indicate, however, that the creatures they adorn, are still low in the scale of organisation, precisely because these colours can thus appear, as it were, unwrought. Here, too, heat seems to contribute much to their development.