A N I M A 
motions necelTary to the animal; and as, when wc have 
recourfe to the microfcope, wc find thole pieces which had 
appeared to the naked eye as (lie primary mechanical cau- 
l'es of particular motions, to confill themfelves of fmaller 
parts, -which are the caufes of motion, extenlion, &c. in 
the larger; when the druidure therefore can be traced no 
further by the eye, or by the glades, we have no right to 
conclude that the parts which are invisible are not equally 
the fubject of mechanifin : for this would be only to af- 
' fert, in other words, that a thing may exid becaufe we fee 
and feel it, and have no exiftence when it is not the objeft 
of our fenfes. The fame train of reafoning may be applied 
to animalcules ; we fee them move ; but, becaufe the muf- 
cles and members which occafion thefe motions are invili- 
ble, fhall we infer that they have no mufcles, with organs 
appropriated to the motion of the whole and its parts ? To 
fay that they ex ill not becaufe we cannot perceive them, 
would not be a rational conclufion. Our fenfes are indeed 
given us that we may comprehend fome efFefts ; but then 
we have alfoa mind, with reafon, bellowed upon us, that, 
♦ from the things which we do perceive with our fenfes, we 
may deduce the nature of thole caufes and effedfts which 
are imperceptible to the corporeal eye.” 
Belides the above fyHematic arrangement of animal¬ 
cules, there has been another, and very lingular fpecies, fug- 
gelled by Leeuwenhoek and Hartfoeker, as pervading the 
femen mafculinum of every kind of animal; an opinion, 
which has alfo been warmly fupported by Andry, Valif- 
' nieri, Bourguet, and other obfervers. The bodies of thefe 
animalcules, fays Leeuwenhoek, are of an oblong oval 
■ form, with long tapering (lender tails bluing from them ; 
and, as by this lliape they refemble tadpoles , they have been 
frequently called by that name ; though the tails of them, 
in proportion to their bodies, are much longer than the 
tails of tadpoles are : and it is obfervable, that the ani¬ 
malcules in the femen of filhes have tails much longer and 
more (lender than the tails of thofe in other animals ; in- 
■ fomuch, that the extremity of them is not to be difcerned 
without the bed glades, and the utmoft attention. The 
number of thefe animalcules, he fays, is inconceivable. 
On viewing with a microfcope the milt or femen of a liv¬ 
ing cod-filh, innumerable multitudes of animalcules were 
found therein, of fuch a diminutive five, that he fuppofed 
at leafi: 10,000 of them capable of being contained in the 
bulk of a grain of land ; whence he concludes, that the 
milt of this dngle filh contained more living animalcules 
than there are to be found people living in the whole 
world. To find the comparative fize of thefe animalcules, 
Leeuwenhoek placed an hair of his head near them ; which 
hair, through his microfcope, appeared an inch in breadth; 
and he was fatisfied, that at lead 60 fuch animalcules 
could eafily lie within that diameter ; whence, their bo¬ 
dies being fpherical, it follows, that 216,000 of them are 
about equal to a globe whofe diameter is the breadth of a 
hair. He obferved, that when the water wherewith he 
had diluted the femen of a cod-filh was exhaled, the little 
bodies of the animalcules burd in pieces; which did not 
happen to thofe in the femen of a ram : and this he im¬ 
putes to the greater firmnefs and confidencyof the latter; 
as the fledi of a "land-animal is more compact than filh. 
Thefe animalcules appear to be very vigorous, and tena¬ 
cious of life ; for they may be obferved to move long after 
the animal from which they are taken is dead. They have 
this peculiarity alfo, that they are continually in motion, 
without the lead red or intermidion, provided there is flu¬ 
id fufficient for them to lwim about in. Leeuwenhoek 
farther fays, that, having examined the femen of a cock, 
he perceived a number of animals fimiliar to river eels ; 
but they were fo minute, that 50,000 of them were 
not equal in bulk to a grain of fand. Of thofe in the fe¬ 
men of a rat, it required many millions to make the 
thicknefs of a hair, &c. This curious obferver was per- 
fuaded, that the whole fubflance of the femen was only a 
mafs of animalcules. He faw thefe animalcules indifcri- 
minately in the feed of men, of quadrupeds, of birds, 
L C U L E. 727 
of fillies, and of infefts. In the feed of a grafs-hopper, 
the animalcules were long, and extremely thin. They 
appeared, he fays, to be attached by their fuperior end ; 
and the other end, which he calls their tail, had a brilk 
motion, like that of the tail of a ferpent when its head is 
fixed. In the femen of young animals, when examined 
before they have any fexual appetite, he alleges that he 
faw the lame minute animals, and that they had no moti¬ 
on : but, when the feafon of love arrived, the animal¬ 
cules moved -with great vivaciy. In the femen of a male 
frog, he faw animalcules ; but, at fird, they were imper- 
febf, and had no motion : fome time after he found them 
alive. They were fo minute, he obferves, than 10,000 of 
them were only equal in bulk to a fingle egg of the female. 
In the femen of a man and that of a dog, he pretended 
to fee two fpecies of animalcules, refembling males and 
females. Having Ihut up the femen of the dog in a fmall 
vial, he fays, that a great number of animalcules died the 
fird day ; that, on the fecond and third day, dill more of 
them died ; and that few of them were alive on the fourth 
day. But, having repeated this experiment on the femen of 
the fame dog, he found, at the end of fevendays theanimal- 
cules as brilk and lively as if they had been newly extradit¬ 
ed from the animal : and having opened a bitch, that, 
fome time before the experiment, had been three times 
covered by the fame dog, he could not perceive, with the 
naked eye, any male femen in the uterus or its appendages ; 
but, by the aflidance of the microlcope, he found the 
fpermatic animals of the dog in both horns of the uterus: 
in that part of the uterus which is neared the vagina, he 
difcovered great numbers, which evidently proves, fays 
he, that the male femen enters the uterus, or, at lead, 
that the fpermatic animals of the dog had arrived there by 
their own motion, which enables them to pals over four 
or five inches in half an hour. In the uterus of a female 
rabbit, which had jud received the male, he obferved au 
infinite numberof fpermatic animals. He remarks, that the 
bodies of thele animals are round; that they have long tails ; 
and that they often change their figure, efpecially when 
the fluid in which they fwim begins to dry up. In the 
preceding plate they are correctly delineated. Fig. 18 re- 
prefents the animalcules in the feed of the rabbit; 19, 
fhews the form of thofe in the feed of a dog ; 20, (hews the 
fpermatic animals in the feed of a ram ; and 2 j, are thofe 
found in the feed of a cock. The whole are exhibited of 
the larged fize they could poffibly be magnified. 
' Thefe experiments of Leeuwenhoek were repeated by 
feveral people, who pretended to have found them exactly 
as above delcribed. Mr. Andry fays, that he could find no 
animals in the human femen previous to the age of puber¬ 
ty ; that they exid not in the femen of very old men ; 
that there are few of them in thofe who are aifetiied with 
the venereal difeafe, and that thefe few are in a languilh- 
ing date ; that none of them appear alive in impotent per- 
fons ; and that the animalcules in the femen of men have 
a larger head than thofe of other animals, which corref- 
ponds, he obferves, with the dgure of the foetus and in¬ 
fant ; and he adds, that thofe who life women too fre¬ 
quently have generally few or no animalcules in their fe¬ 
men. 
The difeovery of thefe fpermatic animalcules was 
thought to throw fome light on the rnyderious affair or 
generation, and thefe minute creatures were imagined to 
be each of them individuals of the fame fpecies with the 
parent. But here the infinite number of thefe animal¬ 
cules was an objection, and the difficulty remained as great 
as before ; for, as every one of thefe animalcules beho¬ 
ved to be produced from a male and female, to ex¬ 
plain their origin by animalcular generation in the lame 
manner was only explaining generation by itfelf. 
The a< 5 t of conception is thus deferibed by the learned 
Boerhaave: The male feed, which abounds with living 
animalcules, animated with great drength, extreme heat, 
and perhaps with a great quantity of animal fpirits, is 
eje&ed with a violent impetuofity through the uterine.ori- 
3 fi ce j 
