Apart from his explaining to popular audiences that whales were mammals and not fishes, Flower emphasised three points very strongly in regard to the organisation and physiology of these animals. First of all, he pointed out that, as a rule, they do not “spout” water from their “blowholes.” “The ‘spouting,’ or more properly the ‘blowing’ of the whale,” he wrote, “is nothing more than the ordinary act of expiration, which, taking place at larger intervals than in land animals, is performed with a greater amount of emphasis. The moment the animal rises to the surface it forcibly expels from its lungs the air taken in at the last inspiration, which is of course highly charged with watery vapour in consequence of the natural respiratory changes. This, rapidly condensing in the cold atmosphere in which the phenomena is generally observed, forms a column of steam or spray, which has been erroneously taken for water.” Secondly, he drew attention to the importance of the rudiments of hind-limbs which occur in many whales as affording decisive evidence of the descent of the group from land mammals. And thirdly, he emphasised the marked distinction between baleen, or whalebone, whales (Mystacoceti), and toothed whales and dolphins (Odontoceti); although he appears never to have gone so far in this direction as some modern naturalists, who are of opinion that these two groups have originated independently of one another from separate types of land mammals. Another point to which Flower devoted a considerable share of attention was the dimensions attained by the larger species of whales. Previously, there is no doubt that very great exaggeration had been current in this respect, and that such things as 150-feet whales are unknown. With his excessive caution, and determination to be on the safe side, it is however probable that in some instances—notably the Greenland right-whale and the sperm-whale—Flower somewhat under-estimated the maximum dimensions.