SB 10.1.51

agner yathā dāru-viyoga-yogayor
 adṛṣṭato ’nyan na nimittam asti
evaṁ hi jantor api durvibhāvyaḥ
 śarīra-saṁyoga-viyoga-hetuḥ
Synonyms: 
agneḥ — of a fire in the forest; yathā — as; dāru — of wood; viyoga-yogayoḥ — of both the escaping and the capturing; adṛṣṭataḥ — than unseen providence; anyat — some other reason or accident; na — not; nimittam — a cause; asti — there is; evam — in this way; hi — certainly; jantoḥ — of the living being; api — indeed; durvibhāvyaḥ — cannot be found out; śarīra — of the body; saṁyoga — of the accepting; viyoga — or of the giving up; hetuḥ — the cause.
Translation: 
When a fire, for some unseen reason, leaps over one piece of wood and sets fire to the next, the reason is destiny. Similarly, when a living being accepts one kind of body and leaves aside another, there is no other reason than unseen destiny.
Purport: 

When there is a fire in a village, the fire sometimes jumps over one house and burns another. Similarly, when there is a forest fire, the fire sometimes jumps over one tree and catches another. Why this happens, no one can say. One may set forth some imaginary reason why the nearest tree or house did not catch fire whereas a tree or house in a distant place did, but actually the reason is destiny. This reason also applies to the transmigration of the soul, by which a prime minister in one life may become a dog in the next. The work of unseen destiny cannot be ascertained by practical experimental knowledge, and therefore one must be satisfied by reasoning that everything is done by supreme providence.