II] THE PRINCIPLE OF SIMILITUDE 21 



has to maintain against the surrounding agencies that are ever 

 tending to dissolve the vital bond, and subjugate the living 

 matter to the ordinary chemical and physical forces. Any 

 changes, therefore, in such external conditions as a species may 

 have been originally adapted to exist in, will militate against that 

 existence in a degree proportionate, perhaps in a geometrical ratio, 

 to the bulk of the species. If a dry season be greatly prolonged, 

 the large mammal will suffer from the drought sooner than the 

 small one ; if any alteration of climate affect the quantity of 

 vegetable food, the bulky Herbivore will first feel the effects of 

 stinted nourishment." 



But the principle of Galileo carries us much further and along 

 more certain lines. 



The tensile strength of a muscle, like that of a rope or of our 

 girder, varies with its cross-section ; and the resistance of a bone 

 to a crushing stress varies, again like our girder, with its cross- 

 section. But in a terrestrial animal the weight which tends to 

 crush its limbs or which its muscles have to move, varies as the 

 cube of its linear dimensions ; and so, to the possible magnitude 

 of an animal, living under the direct action of gravity, there is 

 a definite limit set. The elephant, in the dimensions of its limb- 

 bones, is already shewing signs of a tendency to disproportionate 

 thickness as compared with the smaller mammals ; its movements 

 are in many ways hampered and its agility diminished : it is 

 already tending towards the maximal limit of size which the 

 physical forces permit. But, as Galileo also saw, if the animal 

 be wholly immersed in water, like the whale, (or if it be partly 

 so, as was in all probability the case with the giant reptiles of our 

 secondary rocks), then the weight is counterpoised to the extent 

 of an equivalent volume of water, and is completely counterpoised 

 if the density of the animal's body, with the included air, be 

 identical (as in a whale it very nearly is) with the water around. 

 Under these circumstances there is no longer a physical barrier 

 to the indefinite growth in magnitude of the animal*. Indeed, 



* It would seem to be a common if not a general rule that marine organisms, 

 zoophytes, molluscs, etc., tend to be larger than the corresponding and closely- 

 related forms living in fresh water. While the phenomenon may have various 

 causes, it has been attributed (among others) to the simple fact that the forces of 

 growth are less antagonised by gravity in the denser medium (cf. Houssay, La 



