What can be observed only by observing its affects on gravity?

Dark matter

Explanation:

Very little is known about dark matter. This is because this matter does not interact with electromagnet waves, meaning it does not emit, reflect or absorb any kind of electromagnetic radiation that we know and is therefore not visible even by telescopes – hence it being named ‘dark’. However,  this matter has been observed due to its gravitational effect it is having on visible matter. This matter is believed to be responsible for pulling on galaxies even when evidence shows that the universe is expanding and therefore galaxies should be moving away from each other. Incorporating dark matter in astronomical calculations helps make sense of the seen motions of the galaxies and other observable matter in the universe because otherwise, things would not add up.

answer: land bridge theory. land bridge theory held that shifting portions of dry land explained the finding of identical species on different continents. plate tectonic theory eventually eliminated the land bridge theory.

a

explanation:

many plants naturally reproduce this way, but it can also be induced artificially. horticulturalists have developed asexual propagation techniques that use vegetative plant parts to replicate plants. success rates and difficulty of propagation vary greatly. monocotyledons typically lack a vascular cambium and therefore are harder to propagate.