Sexual reproduction is referred to invariably and implicitly because sexual reproduction contribute towards evolution and variation
as during sexual reproduction there occur crossing over which causes mutation and lead to evolution and variation which some time proves to be good for the survival and gives better chances of sustaining in a changing environment and sometimes resistant against disease.

Explanation:

The statement "Invariably and implicitly we refer to sexual reproduction" means that whenever we talk about reproduction, we are referring to sexual reproduction, even if it is not explicitly stated. This is because sexual reproduction is the most common and natural way for organisms to reproduce and is a fundamental biological process. It involves the fusion of gametes (sperm and egg) from two different individuals, resulting in the formation of a zygote, which develops into a new organism.

Importance of Sexual Reproduction:

Sexual reproduction has several advantages over asexual reproduction, such as genetic diversity, better adaptation to changing environments, and the ability to evolve and develop new traits over time. It also plays a crucial role in the survival and evolution of species by allowing for the transfer of beneficial traits from one generation to the next.

Examples of Sexual Reproduction:

Some examples of sexual reproduction in nature include:

- Mammals: Mammals reproduce sexually, with the male fertilizing the female's egg to produce offspring.
- Birds: Birds also reproduce sexually, with the female laying eggs that are fertilized by the male.
- Plants: Most plants reproduce sexually, with the flowers and pollinators playing a crucial role in facilitating fertilization and seed production.
- Insects: Insects such as bees, butterflies, and moths also reproduce sexually, with the male transferring sperm to the female during mating.

Conclusion:

In conclusion, sexual reproduction is an essential and fundamental biological process that is inherent in almost all living organisms. It allows for genetic diversity and adaptation to changing environments, ensuring the survival and evolution of species over time.