1831 .] 
On Varieties in the Animal Kingdom , 
81 
substance, or in spirit ; but it cannot belong to the first class, otherwise it would, 
at one time or other, be evident to some of the senses ; and if it belongs to the 
second, what can possibly be the object of its mission, attendance, or presence, 
unless to influence the actions of mankind : yet, if this be admitted, which cannot 
be denied, without destroying the only evidence offered to support its existence, it 
is inconsistent with free agency, and this is the doctrine of necessity ; the merits 
whereof, however, being foreign from the subject of this essay, it is not intended to 
discuss, having made the deduction only to prevent a negligent incongruity of 
diction with sentiment, as it is much suspected those Avords are often employed, 
without any precise or definite ideas annexed to them. If, on the other hand, 
they be supposed only affections or qualities of individuals ; the two opinions, 
when examined, will be found, like neighbouring streams, converging towards the 
same pool, either to unite upon the way, or having finished their courses separate- 
ly, to become stagnant in the same place 4 . 
To return from this digression : the progressive motion takes place, when the 
middle points, at both ends of the cue, and the centre of the ball makes part of a 
right line at the time of impulsion ; and if this position be also parallel 5 with the 
table, as a (see Jig. 1.) it will be smoothest, lightest, and least impeded. For should 
the cue be pointed downwards, as b, the motion of the ball must be partly destroyed 
by reaction from the table ; and, if pointed upwards, as c, or any where else, the 
power of gravity would be more directly in opposition to it, and, contracted at the 
same time within narrower limits, as d, e, still further in effect augmented. But, 
when pointed horizontally, that power acts against the ball at right angles only, 
and being extended, by a stroke of no more than equal violence, between limits that 
embrace a greater latitude, as d, f it is weakened by diffusion, and, if the impetus 
be considerable, will be almost lost in the minuteness of division ; till by continued 
operation, together with the resistance arising from the roughness of the cloth, and 
opposition from the air, the volocity of the ball becomes so mnch diminished, that, 
though at first it scarcely touches the surface, it afterwards (more emphatically) rubs, 
and at length constrained to roll, (as the only means of discharging the progressive 
force which remains,) it thus advances to the place of rest. 
[To be continued.] 
IV . — On Varieties in the Animal Kingdom , depending upon procreation 
between individuals of different species. 
[From Meckel’s Comparative Anatomy.] 
When procreation takes place between two beings of different species, the pro- 
geny is said to be hybrid, and is denominated mule, or bastard; the latter of 
which terms will be employed in the folloAving observations, as the former is 
generally used in a more restricted sense, being applicable to a particular hy- 
brid animal. 
4 Tbe frequency of repetition, and extraordinary remarks, besides the absurdity and 
inconsistency so common in the use of these words, it is hoped will palliate this di- 
gression. 
6 T his stroke is seldom completely necessary ; and, as it cannot be given Avith any 
part of the cue over the cushion, nor very conveniently in any situation : with the 
but end held so low it is seldom made. 
